Here's a little thing I found just now that might be helpful in the future:

"The easy way to tell the difference was that you could lay a straight edge on the machined surface of the crank and rod throws. The 305 crank would allow the straightedge to lay flat across five points,the 350 crank wouldn't. The difference in balance was in the mass of the counter weights,these engines were internally balanced[info from john lingenfelter]."