Later model 3.8 swap in an "A" body

Don't forget about the imports, a BMW engine and trans might be pretty cool, like from a M car

But yeah, put in a small cam, some 2.73 gears and a 500 cfm Eddy carb on a 318 with high compression pistons, you might get 20 mpg.

Seems like alot of work putting in any make V6, by the time its done you might regret not just buy the whole car, lol