If the 305 and the 350 have the same stroke why would they need to be balanced differently?
For one thing, over 1/4" difference in bore size, resulting in a noticeably different "reciprocating weight" when computing the bobweight used to simulate the weight of the parts attached to the crankshaft. The usual rule of thumb is to clamp a weight onto the rod journal equal to 100% of the reciprocating weight and 30%-40% of the "rotating weight" (lower third of the connecting rod, bearings, etc., plus a fudge factor for oil trapped in the assembly. Then the crank is spun and weight is added by welding or removed by drilling the counterweights, similar to balancing a wheel.
One interesting bit of trivia about inline 6-cylinder crankshafts is that they can be spin-balanced without bobweights because of the 120-degree separation of the counterweights. For a tight-winding inline 6 race engine, I computed the bob weights anyway, but for a low-RPM street engine, it really wasn't necessary. A 4-cylinder, V8, or V6 crankshaft will jump out of the machine if that is attempted!
Jerry
