I am not as strong in the eco side of things, but I sell HP speed stuff consistantly.
Here is my recipe:
- Keep the straight 6
- Give it a quality tune up, perhaps adding an upgraded CDI ignition box, timing advance to highest point engine will allow without pinging.
- Change all fluids, synthetic in trans and rear end
- repack wheel bearings, synthetic grease
- evaluate current air filter housing....add K&N filter or equivalent, and look into fabricating an air intake. Hot air is best for MPG and warmup, so you could look at capping the end of the stock intake, and draw air from the 'warmup' tube that normally sits off the exhaust manifold. If yours is not setup this way, it could be made to draw air directly off or around the exhaust manifold, for warm air
- exhaust, have moderate diameter (2" or 2.25) exhaust run, as smooth as possible, with a low restriction muffler
- electric fan to replace the mechanical fan
The biggest detriment to your MPG is gears and transmission. The transmission is a non overdrive 1 to 1 ratio. Without overdrive, the car will likely have an ideal conditions around 50-55 MPH. It will go higher, but the sheer number of RPM will hurt MPG regardless of how well tuned the car is.
The rear gears contribute to this. For max MPG you want the smaller number gears, but accelleration off the line suffers. A 2.76 or so ratio will extend the highway happy place higher, but not the same as an OD trans.
Ultimately, I think you best bet is to get the car in great state of tune. Have the carb rebuilt if it is out of your area. Fresh ignition parts, premium quality stuff, plus any little tricks you can think of (indexing the plugs, etc). Reduce intake and exhaust restrictions, reduce drag and friction as much as possible, etc.
The suggestions for driving technique are very valid. Pulse and Glide is key. I have strategic areas on my daily commute where a P&G yields me a long stretch of glide, with the car idling at its lowest point.
A secondary alternative, is a swap to a modern EFI engine and trans.
A 5.0 and 5 speed from Mustang would give more power and torque to get you moving, and the 5 speed would drop RPM on highway for MPG.