Bone Stock 64 Malibu Conv

[carpoor]

Those 283's can be a real gas sipper with the correct set up. I would put a TH350 in the car, direct bolt in. Then swap in an RV-type cam for more low end, a 4bbl intake, reworked Quadrajet, a set of Tri-Y headers, and a nice dual exhaust system (2.25") with an X crossover. Simple, low buck, and around 19-21 MPG easy.

If that won't cut it, look for a newer truck (99'-07') 4.8L V8 and overdrive automatic. I knock down 20 MPG mixed driving in my daily driver 2000 GMC Sierra. My truck is about 1000lbs heavier than your Malibu and less aerodynamic. I'm sure a 4.8 in a lighter vehicle could easily yield 25+ MPG, and it runs on regular gas. The 4.8's are rated 255-270 HP in stock trim, they make decent power for a "truck motor."