You'll need to turn the main bearing journals down first, or open the block up to the 360 size. I know you weren't looking for a pissing contest or anything, but this seems to be a lot of custom work to make it happen.
Yeah, that and the piston weight difference means $$$. You would have to cut the crank mains and rebalance for the 318 pistons. It just isn't worth the money. Drop in a stroker assembly and do it right.