Would you have some reference to back up the zero balance specification? All the information I’ve ever read or heard, states that ALL 221-260-289-302 were 28 oz-in. external balance, while the 5.0 is 50 oz-in. external balance.
The reason, as I understand it, is that with stock crankshafts, there is insufficient room in the crankcase to have an internally-balanced crankshaft. If you use an ultralight competition crank and reciprocating assembly then you can get the counterweights small enough to go with zero external balance.