I can not add a whole lot here, as I have spent more time behind a flathead than I have a slant 6. My PERCEPTION is that a slant 6 will tolerate RPM with more ease. My 230 flathead is happy and bomb proof between idle and maybe 2700RPM. Yes it can be made to turn more RPM's but a slant 6 will live and thrive where the flathead tends to run out of breath.
I will agree the Slant 6 and the Flathead 6 are 2 different animals.
I.would agree they would perform optimally at 2200 to 2700 cruising speeds all day long. Some flathead 6 owners cruise at 3000 rpm on highway.
Stock they are rated at 3600 rpm, and will perform in the 3200 rpm band if needed.
Chrysler would routinely run these engines for 50 hours at 3600 rpm for tests during the production years
You want these engines to breath snd perform better, same stuff as slant 6 in theory.
Yes a flathead can be made to rev to 5500 rpm, would they run like that all day of course not. A 225 slant 6 would perform better than L head for sustained RPM perhaps, although a 225 has a 4 1/8 stroke and is an undersquare engine, so piston speed is a concern.
Flatheads can certainly be made to perform, to the degree of a slant 6, no as they certainly are a different movie.


The 265 has a 4.766 inch stroke and is rated as 228 Lb.Ft @1600 (according to my Motor's manual)

If you are want to keep up with traffic, and drive the truck at the fast pace of today, then a slant may be happier singing along at 3500 RPM

If you are wanting to cruse and loaf, and enjoy the truck at the speeds it was built for, then a flathead will serve very well. With an overdrive, and the 265, with some modifications, it may be very nice as long as you are not trying to spin the motor real hard.

I am well familiar with Mark Goodman and Rusty Heisten's 62 "Team Green" Lancer. Seen it run many times and always in awe of their work and the car's performance.

Of course, any Slant 6 is super different these days, and you can easily make a bunch more power than a flathead with less work. I have 3 NA Slant engines (two running in cars, one waiting for body repair on 3rd car) that make north of 240 HP and I drive them everywhere. My turbo EFI Dart makes 350-400 HP and gets 24 MPG on 75 MPH cruising. I don't bother screwing together a Slant that makes less than 180 actual crank HP (always drag tested) these days, and that is relatively mild in my book. Just for reference...

Lou

The 265 has a 4.766 inch stroke and is rated as 228 Lb.Ft @1600 (according to my Motor's manual)

Yes, the stroke of the 265 is like a Cummins 6BT (4.72-inch) and the RPM it favors is similar to that widely admired diesel. George Asche says that if you pay attention to the oiling system the flathead can do sustained higher RPMs, but I don't think they make much power above 3,200 rpm. They were also used in tractors and combine harvesters. The cylinder walls tend to be thick so you could bore the 265 to 283 inches and get more torque that way. The Hudson 308 has a 4.50 inch stroke and it won some races.

These are mid 1930s technology that was an improvement over the '20s. By that time the metallurgy had advanced so that a high speed shorter four main bearing six was practical, which eliminated the need for the then very heavy seven main bearing crankshaft, and with the 23-inch flathead, it was compact enough to fit in a space not much larger than the four. The costly long bendy crankshaft is a reason Ford didn't have an inline six until 1942, why he risked building his v8. Chrysler was trying to give Plymouth buyers a six for a similar cost as a four, but he would have preferred to keep building fours for Plymouth, but in 1929 Chevy began the horsepower wars with its low cost straight six. The 23 inch six was built into the 1970s for industrial uses. The seven bearing six eventually reached 413 cubic inches and was replaced by the 354 hemi for use in heavy duty trucks .

It is relatively easy to build a 150 hp 265, it costs a lot to build a Ford flathead 8 that makes that much and is reliable, but the classic Ford 8 sure looks cool! There are now more Mopar flatheads than Olds, Studebaker, or Pontiac etc. But the new aftermarket support for the Ford never ceases to amaze me. The overheating and cracking problems of the Ford can be fixed now, if you've got money to spend that's where the L-head hobby is. I'd sure like to see more interest in new parts for the iconic slant six, but I doubt it will ever happen because the \6 was marketed as an economy engine when it was new

Question,
Did those Chrysler flat head six’s have cast iron pistons?
And if so, is that what is still used on modern rebuilds?
Asking as I think that the IH inline six cylinder motors ( Red Diamond, Silver Diamond) had cast iron pistons,,.
But I am going on 50 year old memories,,,.