Is it possible to do this on a 5-plug block? The exhaust manifold covers up the top half of the plug. There is about 1/4-1/2" clearance between it and the plug. There's no way to get a socket on it. Would a straight edge going across the plug (parallel to the manifold) be good enough?

Those rubber ones are another option but I'm not sure how well they'd survive with the exhaust right there. The stud may hit it.

I suspect my overheating condition is due to a plugged block. The block drain by the oil filter was plugged solid until I dug around it. Just trying to limp this block along until I can sort out a full rebuild it needs.

I would take the water pump off and probe the water jacket from there before I pulled the freeze plugs,,,are you 100 percent certain the radiator is clear and flowing properly? Overheating is usually more of a radiator coolant and or air flow problem than a block-head issue.

Getting to the lower part of the block via the core plugs is not going to work too well IMO with the exhaust manifold on place; the crud is down below the plugs and you're going to have to angle down to get at it. Just remove the manifolds ot make life better. And you won't have any success getting to the passenger side of the water jacket or inside the head. Only removal and acid cleaning will do that.

I had the same issue on a '62 block; pulled the block drain and nothing came out! I flushed and cleaned and flushed and cleaned with a variety of mixtures and methods but never got it out completely. However, I did not physically scrape the water jacket like you are trying to do. Once the block was cleaned as much as I could and then run for a bit, removing and back flushing the radiator and tapping it to loosen the resulting block crud in the tubes helped get the temps to a tolerable level.

You might help yourself to put a screen in the top radiator hose to catch the loosened crud that IS going to come out of the block, and take that screen out regularly for a while to clean it out.

I had replaced the water pump previously and it looked clean from what you can see while motors still in car.

Looking through the cap the rad looks fine. When I bought the car a few years ago the coolant looked very bad, chunky and rust colored, couldn't tell what color the antifreeze was. After flushing it looks fine now. Whether any gunk is lodged deep in the radiator I couldn't say. It must have had overheating issues before it got to me because it had no thermostat in it.

I'll have to poke around with the IR thermometer to see if there's any rad blockage you can't see

Decades ago, in the early days of the Slant-6 Club of America, the magazine contained an ad for a clever tool the maker/seller called the "RIPI", Right-In Plug Installer, which dealt with the awkward angles involved to install a freeze plug in a Slant-6 engine with the exhaust installed. Google search turns up nothing.

You may very well be right about your block being crudded up with mud. Time for some disassembly!

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The head has a stuck exhaust valve on #6. Compression is down on it 40 psi vs the rest until you tap on the valve and it instantly comes back up. If head's coming off project creep will push to get it rebuilt.

Taking the manifolds off is do-able. If I can't manage tapping the plug in evenly or the rubber ones don't work out that is an option. My bearing race / seal driver kit has a small size good for the freese plug but of course you can't get the shaft on it or tap on the top.

I managed to get a rubber plug to work by shortening the stud and the rubber part then reassembling it. It still works, but I found it actually took less time to pull the manifolds and do it with a regular plug. I spent more time digging the old one out.

The radiator doesn't appear clogged. I measured with the IR thermometer about 1/3 the way down from the top tank and the temps are within a degree or two across the rad. That's within measurement tolerance I imagine.

Down to the head or block at this point I think. I almost wanna tear into it just to see what the heck is in that thing.

I see a definite drop through the rad from upper hose/tank to lower hose/tank of many degrees; can't recall the exact numbers but it was more than a few degrees. I don't believe you can really tell individual tubes temps from an IR gun; the IR spot size is way too large. So, I don't feel that is an adequate test for individually clogged tubes.

The one thing about mud in the bottom of the block is that the coolant flow can go mostly above that and flows through and up to the head. And the heat is least in the bottom, so while the mud is a bad sign of long term corrosion and perhaps broken down old anti-freeze crud, it may not effect cooling so much. Circle track racers commonly fill the lower ends of cylinder blocks to reinforce between the lower cylinder walls and keep the engines cool enough.

The other thing to consider is that the head and block have some matching water jacket holes that could support coolant flow in the front and middle, but the head gasket is solid there and the closes those off holes to force more flow towards the rear. The original steel gasket can corrode and break through there and allow the coolant to partially short-cut the flow path rather than going mostly to the back cylinders and then up through the head in the rear.

I went down the same road.... had the sticky valves too like yours. Finally just pulled the engine and rebuilt. It was worth the time and $$ to me to have it right and the degree of heating on the interstate is normal now.

Oh sorry should have clarified. Top to bottom on rad there is definitely a drop, rad is working. I was measuring left to right to verify sections of it may be clogged. But your right, if something like every other tube was clogged you couldn't tell that with an infrared gun.

Temp issues aside, the sticky valve and low oil pressure (10psi hot idle, 10w30) is reason enough for the rebuild.

This is really bad timing since I'm out of work but yes, I'm going to bite the bullet and start planning a budget build.

steel-brass is a pretty good conductor of temperature,, if every other cross tube in the radiator was blocked, or if you all were blocked by 50% you would loose half of the radiator cooling, but the temp across any couple of inches of the radiator could be about the same,,,

the cross sections in the radiator are going to be a lot smaller than the cross sections in the block - head. So if you think the block and head have corrosion blockage, but the radiator does not,, I question that. So if you are going to tear in to the block and head and do a rebuild, and clean out those water jackets and not address the radiator that had coolant in it that you could not tell the original color,, think about that.

I would get the radiator to a quality shop for coreing-cleaning, what ever they do on radiators these days, get the radiator in good shape,,flush the block aggressively, best you can without a major tear down, and see if that buys enough time to save for a proper rebuild,

Ow, sorry about the out of work. Well, at least you have time!

FWIW..... If $$ were short for me, I would take the rad out and back flush it for a long time and tap and tap and tap on it to loosen up as much crud as I could; you may be surprised how much crud you can work out that way. I did that for 30 minutes on a plugged Saab rad (to much stop-leak!) and quite adequately freed up the flow through in it that way, and got a lot of crud out of my '62 Dart rad last summer with that method. I also blew shorts burst of compressed air in with the water flow to try to shock some crud loose. (Some claim that you can rupture a rad that way so be cautious.)

And it sounds like you have the time to do some block water passage scraping. I'd do it with water running into the pump hole and the block plug out and the core plugs out if you take them out.

Then work some carb gum-out down the valve guides to help that and put in new valve stem seals while you are at it.

The oil pressure sounds liveable if it is 10 psi hot.

Do you have the compression readings? And have you run a cooling system pressure test to see if you have compression gases getting into it?

Sounds like you might avoid a rebuild for the sake of $$ at this time if a full battery of tests hows no compression issues or leakage into the water jacket.