318 or 360? That is my question...

Really it depends on "what you find" and "for how much."

So far as swap the big thing is the flywheel balance. You must use the 360 matching balancer, and you didn't say whether you have a stick or auto

If you have a stick you need a 360 weighted flywheel

If you have an auto you can buy aftermarket (BM) weighted flex plate for the 360 balance.

I cannot imagine, though, why you are having so much bottom end trouble.