340 pistons

I don't know what engine he is replacing but if it's a 318, I'd use it's crank, whatever pistons you decide to use, and the 318 balancer and converter/flywheel. Those externally balanced 340's are getting harder to find certain parts. Making it an internally balanced engine will make parts swapping a whole lot simpler and cheaper. You could sell the 340 cast-crank-only parts to someone doing a restoration and help fund a decent rebuild.