I know that the cast cranks for the 305/350 2 piece seals was 3932442. The easy way to tell the difference was that youcould lay a straight edge on the machinedsurface 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]. The 86-late 305 /350 shared the same one piece cast crank 14088526. I can find no info as to whether the crank throws mass was different on this crank , as it was externally balanced. Does anyone know ?

 

Might be some info in this thread for you, look for RB's reply.

https://www.thirdgen.org/techbb2/sho...hreadid=151311

 

all SBCs are internally balanced, except the 400. it gets confussing because of the bat wing weight and the dowel pin to index the flywheel/flex plate. basically an internally balanced engine will have a counter weight the correct size or mass to off set the mass of the piston/rod/pin assembly. on short rod engines, like the 400 there isn't enough space for the counter weight to be of correct size and still clear the piston. the bat wing weight was added to replace the mass removed from the flywheel flange when gm went to the one piece oil seals.

 

The machine shop supplied the cast one-piece seal 305 crank, .010/.010 and I can't remember for a fact that I check the crank to make sure it was a 305. I seem to remember giving it the straightedge test,but I'm not a 100% sure. Anyway I assembled it using p.m. rods[which were all within +/- 2 grams] and ft. trw forged pistons. Using a mild hyd.ft.cam it won't ever see over 6000 rpms,was concerned about balance problems.