My son, Dave (a Ford Factory Trained Master Drivability Tech) and I install lots of mods on cars, everything from Superchargers to the smallest part to get performance out of a Mustang.
The cheapest seat-of-the-pants performance part is to install a set of 373 or 410 gears. Gears less than $200 and labor less than $200.
I'm running 410's with a Powerdyne SC.(plus lots of other mods), on my '95 302 and on the Hiway, I average 20+ MPG. Of course around town, I don't get that, but I don't care. I like being pushed into the back seat on launch.