[Alex V.]

With that trans./axle/tire combo you're going to be turning 2,000 RPM @ 60 MPH, so to have an engine that gets good mileage running at that RPM range most of the time it may not have a lot of seat-of-the-pants power in the upper end. Not that that's a bad thing, you just may not be able to have your cake and eat it too. The L31 Vortec crate engine may be a good choice - even from the factory (albeit in fuel injected form) they have enough mid-range power to move a truck along well while getting 20+ MPG in some cases.

Off the top of my head, if tuned well I'd think a Quadrajet and Edelbrock Performer? dual-plane intake would be a cheap and effective combo for pulling good mileage out of a stock L31.