400 B block

If MM is selling a crank cheaper, it's not a US part either. Ohio crank, Eagle, Scat, 440Source, even Callies, are all using the imported stuff now as thier cheapest lines because cost is ALWAYS the first question. If it's a bracket racer, I would go the B rod (6.535) and the 3.915 stroke. Run a head that makes the power you want in the rpm you want, and it will last for years. A piston shorter than 1.45 compression height is asking for wear and stress, plus, you add a lot of friction thru side loading. IMO, it's just not worth it. A properly designed race engine can go beyond 800hp in that configuration on race fuel. The RB is taller by a little less than an inch IIRC. That allows for a taller piston for any given stroke/rod combo.