There are used good 5.0's around in $500 range. I'd look for one from a 5.0 Exploder (Explorer), usually a pre Feb 97 will have GT-40 heads & the cast GT-40 intake is in the same class as a Cobra, has a 65mm TB, all good starts. Most 5.0's even beyond 100k miles are good swap candidates.
You'll need to change to a TFI dizzy, Explodersusually have a crank trigger. Then go to james W's site, every thing is there, or for a specific answer just ask.

My 5.0, EFI, AODE-W, 3.25 diff, can get 28mpg @steady 65mph, average is prolly20-22, Interstae @75-85 is 22-23. T-5 tranny would do as well or better.
I'm in the process of adding bolt on's to my EFI(bone stock+K&N, short belt driving only Alt, cast iron 289 manifolds) If anyone is interested, I'll post results of individual changes.

 

I paid 700 for mine. Kind of high, but I knew it's history and it only had 35K miles on it. Never been broken into. Still had the factory stickers, wires, and plugs.

There are deals out there if you're patient...