Gas mileage? 225 stock cam 30 over machining rebuild, points versus electronics distributor? And the best set up for aftermarket cruise control, best gear ratio? Right now I have Dual headers and it Offenhauser intake with an elder Brock 600 4 barrel carburetor.

Too much carb for a stock slant. HEI electronic distributor will be way better than points. Gear ratio?? Is it a truck? dart? ???...

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Slant power!!!
At least I try...

It’s s A108 Dodge van

Why electronic? What’s the data ? What carb ? That goes with an Offenhauser 4 barrel intake

Not just electronic.. Up grade to a HEI system. Look into the articles section and you will find an explanation as to how and why.
Gears,, I'm not an expert, (please correct me if I'm wrong) 3.55 or 3.5 rear would be good for power and gas mileage.
A two barrel carb would be a better choice for stock, you will need a 4 to 2 adapter in order to mount in on the intake you have. ( Easy to get on line or make).
Many will recommend a BBD, Weber,.. You can also find an article on this too..

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Slant power!!!
At least I try...

The 4 barrel is only a two barrel until you smash it, right? If you have it and it is in good shape it can be made to run just fine. The 500 boosters may be better fit, but we have had several 225's running with 600's on them without trouble.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

It will be difficult to get good fuel economy with a van because of its massive wind resistance. Not exceeding the speed limit will help greatly. Choosing the right axle ratio is a trade-off between acceleration and fuel economy. Lower numerical axles have better steady-speed fuel economy because engine RPMs are lower thereby requiring more throttle. More throttle results in lower manifold vacuum which in turn lowers pumping losses. However, depending on how your carb is setup, lower manifold vacuum can cause enrichment, which would reduce your fuel economy.

There is no reason that you could not get good fuel economy with an Edelbrock 600 CFM (Performer?) carburetor although the Holley 390 CFM carb is better sized to 225. I had a 625 CFM AFB on my car for many years and had it turned sideways so the throttle valves were parallel to the engine and the primaries were mounted farther away. It worked OK but had a persistent stumble on left turns. It also had a stumble on acceleration that was resolved with the addition of a water-heated hotspot under the carb.

I would say that an HEI upgrade is easier to do than a Mopar EI upgrade. It also has a hotter spark, which will allow you to run leaner fuel mixtures. I'm working on improving my highway fuel economy but didn't get much driving in last year after the HEI upgrade. Previously, with the Mopar EI and the Quadrajet tuned lean, I was easily getting 25+ MPG (US) on the highway.

See Quadrajet Upgrade, Quadrajet Tuning, and HEI Ignition Upgrade.

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1965 Plymouth Barracuda,
225 engine, Quadrajet, HEI, Dutra Duals, 904 Torqueflite, 2.76:1 axle, Addco front bar
Rods & Relics - Fort Erie, ON / Collector Car Tech

Great stuff thanks

I have been looking at distributors , and I’m having trouble deciding on what to buy? EHI but what brand name, could you give me advice on what you think would be good for what I have ? Stock cam 30 over stock motor ? Thank so much

You need a non lean burn Mopar Electronic distributor. Then perform the HEI mod using a GM HEI module in place of the Chrysler ignition control box. I managed to find one with a 9R governor that limits centrifugal advance tp about 12*. There are other desirable governors but I don't know the numbers or year models for these. Diligent searching here will yield that information.

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“Buy the ticket, take the ride.”
― Hunter S. Thompson

My only suggestion is to re-gear the vehicle so that you will be at or just below peak torque while at the normal cruse speed...

My Gas Mileage experience is as follows: I made a 1400 mile trip in a '84 mercedes diesel wagon. On the way out (700 miles) I was going at the speed of traffic most of the way. (65-70 mph) @ 28MPG

On the way back I was driving at 2400 RPM as much as I was able... which was between 52-58 MPH depending upon the hill. I was driving at that speed b/c it would run hot when forced to keep up with traffic. 34MPG.. YES some will claim that I was going slower, and that is a factor, BUT combustion engines are generally most efficient at peak torque. I later found out that 70 MPH in that vehicle was 500RPM over peak torque.. Thus combustion efficiency was lower.

So I am of the belief that being honest about how fast you want to set the cruise control at (or you right foot) and gear the differential (and transmission) to be operating at or just below Peak torque, and you will have the engine using the least amount of fuel for the work it is providing.. All other enhancements will add to this...

Edit: Just noticed you are in a A108... So there may be the factor of wind resistance to consider.. That is to say if you were to follow my advice 100%, and re-gear the van with highway gears.. it is possible that the combination would be lugging most of the time when on the open road, which will not help your fuel economy.. (and make it feel underpowered) I would make all of the other improvements first, before the rear gear change... And experiment with driving at peak torque on a trip, so as to find out what is possible with the setup. Then consider a gear change or an overdrive transmission..