Uhmmm, wow, that's a tough question....
There are a few ways to go about this project...
First and probably cheapest... find one that someone else has already built and needs to sell.
It's almost always cheaper to buy some elses finished project than it is to build your own, just nowhere near as satisfying to look at and say to your buddies, hey, I built it!
As far as going to a newer more fuel efficient engine and better gearing for highway/ freeway use...
Fuel injected 5.0 from any later 'stang t-bird, Lincoln etc...
5 speed manual o/d tranny (several to choose from)
4 speed o/d auto trans, again there are a couple to choose from.
Smaller tires will increase fuel mileage, driving habits are probably the most important factor in determining actual mileage. Keeping the engine tuned properly, and tire pressures at optimum for your vehicle are also important.
My non fuel injected 69 Bronco on 35" tires gets combined mileage around 21 mpg if I keep my right foot under check...
Do some surfing and some more research, you will find many, many EB builds on the web, there is plenty of information available, both what to do, and what not to do.