318 with 340x heads sluggish

It takes about a .070" cut to equal one compression ratio. You'll have to mill over .100" to get compression on the high side of the 8s. Then you'll have to have the intake milled to match. That's a LOT of cuts for the machine shop, because they cannot do it all in one pass. It's not gonna be cheap, plus, you'll have an intake that you can ONLY use with THOSE heads. Get a clue. That's not the right way to do things. DO it right, or take up solitaire.