What would you do?

I agree with most here. Leave the 318 as is, but I would beef up the exhaust a bit (68'- 71' stock manifolds) or a set of headers. Build another "HOT" motor if needed for special moments(a one day swap out), but if you want to get a little more serious, a 727 and 8 3/4 may be in need. Now your getting into heavy springs, cooling, tire upgrade and such. Back to "leave well enough alone", nice car as is.