You call Smith Bros and they ask length? pressure open? Power adders? then they tell you what wall thickness AT THAT length will work --Monte is correct--that is all there is to it
I am guessing Cab found 8 more HP because the rings were seating?? No snub to Cab I think he knows his stuff and I could be way off so...no harm meant just asking
I know longer push rods like on an Indy head etc is the length where yea maybe that 5/16ths is not the way to go AT THAT LENGTH
I get that