I don't know a whole lot about the small blocks, but I would imagine that everything would be the same except your 318 might not be internally balanced, whereas the 340 should be, so you would actually REMOVE weights from your torque converter. If it were my car I would get a torque converter that is designed for an internally balanced engine. Be aware the heads and intake are different on a 318 than on a 340. You can put 318 heads/intake on a 340, but why would you? I don't know if you have performance heads/intake in which case you have already taken off the 318 stuff and put "340/360 heads and intake" on.