67' 318 crank in a 69' 340

One thing to do to figure out a combo is come up with some solid numbers on how far off (overbalance?) the bobweight can be. A one percent OB would be what, 20 grams? That allows some fudge room. I am not a balancing expert so don't go by my 1 percent idea. But there must be someone on this board that has an idea of what can be done.