It'll cost you a few bucks, but surely an early 3.3 L6 Mustang and a four-speed overdrive SROD conversion, or even a T5. Very hard getting four-speed bits for the big bell engine, and all emmision nazis know 200's in the Fox 'Stang never had four-speed manuals. (Option II , God forbid, the 2.8 V6 which came stock with the 2.3 style Holley Weber on early pre 78 cars. The Motorcraft 2100 2-bbl carb can be a good swap if it came with the nasty variable venturi carb). The Cologne V6 can be thirstier than the 3.3. There was little low end torque.
If it were me, I'd find an unloved but mint 2.3 liter 1980 'Stang in mint condition with the stock 2.3 T5 manual, and then wack in the six cylinder cross member and 3.3 engine. Use the carb that came with the 2.3, and then place an '80 3.3 engine in it.
The diff is the issue. 3.45:1 gears, and it would be cool. As soon as you've got the normally standard C4, you've got problems with moon shoot diff ratios in the 3.08 or 2.73:1 area. If the carb is well sorted, it should give excellent economy.
But to get anything like good figures, you're going to have to do some tuning, my friend. Despite what your Mom says, keep off the cologne, and saddle up into a bigger pony.
As for the Ranger, a great little truck, but blunt faced trucks cost fuel through lousy aero-dynamics, weight, and gearing that is made for good free way figures, but lousy in stop and go situations. Unless you're going to use the tray, forget it, and have a lighter Stang instead.
