Since inline six engines are naturally balanced by the degrees of rotation between the cylinder pairs, as long as the weight of the rod/piston assemblies are properly matched, they can spin to some unbelievable speeds. The limiting factor on a stovebolt engine will always be the cylinder head. Port size and shape, and the route the gases must take getting in and out of the engine will always be the bogeyman the stovebolt hotrodder has to deal with. The obvious solution is to build for low and midrange torque, not high RPM. A stroker crankshaft gives the engine a longer lever- - - -more torque, especially down low where it's useful.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!