318 vs. Slant value

body work is expensive and difficult to me. If the slant has a great body, I'd start there. Once you make the swap and assuming that you do the same brake/suspension work to each, I don't think that the original configuration will make a bit of difference. What you end up having is a great hand crafted, well thought out hot rod... so let's see some pics of the candidates - sounds like big fun to me!