I'd bet that for every flathead Mopar 6 that's been built, 1,000 Ford flathead V8s have been.
I would think a lot of the same tricks would apply, plus the Mopar probably doesn't have the heat issues of the Ford V8.
A racer buddy of mine does single-cylinder (Mustang scooter) motors and gets quite a bit more out of them, and there's almost zero HP parts for them.