Please don't kill me...

Mostly it depends on your budget and how much of it you can do yourself.
What you need is a peppy little injected small block with an overdrive trans, but what you can do might be another story.
Guys here love to spend other people's money sometimes.

I have about 3k into a mild 350hp or so 5.9 Magnum small block and a 5 speed OD trans, but I was able to do all myself.
25mpg on 87 fuel and 2,000 RPM's at 85-90 on the freeway.

It won't save you from the Wife that wants 40mpg and no women looking at you and your car though. :D