Biggest difference would be the foundry......

R1 was done on production line tooling, R3's are done by Motor Castings Co in Milwaukee. "Generally" the R3's have much better core shift/sonic #'s because of this.

No R1's were avail in siamese bores... R3's avail in both siam & no siam

R1 will have liter valley that more closely resembles a factory valley, will have oil drainbacks & center will be open to cam tunnel, R3's fully filled in cam tunnel with no drains out of box. R1 will be around 20 lbs lighter than similar R3 due to difference in valley material.

Both will have 18 bolt head provisions & +.400" tall lifter bosses.

Bottom end/Deck thickness etc, pretty much same on both blocks.