The 4 3/4 crank used in the 265 is pretty well the limit. They even had to use special rod bolts with "no heads" to get them to clear the block.
The flatheads were made to be rebuilt several times during their service life so a 1/8 overbore is not out of the question on a good block.
Personally I think boring and stroking this engine for more displacement is futile.
I agree you can have a lot of fun pepping one up with a cam, carbs, split exhaust etc and not spend too much money.
But if you want any serious horsepower, forget the flathead and get a bigger OHV engine.
If it was my engine I would do a straight rebuild of a 251 or a 265 if I could find one. Possibly add a cam, dual exhaust, twin carbs, and electronic ignition, mill the head and let it go at that.
O ya shave the flywheel, this is an old time hot rod trick now forgotten, that adds a lot of snap.
These mods should give you around 160 or 170 HP and a good performing car.
If you really want substantially more than that, forget the flathead.
I should also point out, for those who are not familiar with the old long stroke flathead engines, that pure horsepower does not tell the whole story.
Their broad powerband and ample low speed torque make driving sooo easy especially in the low to medium speed ranges. And the old Mopars are surprisingly light in weight.
A good flathead is a lot of fun to drive and does its work a lot easier than you would think.
