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I worked on Mustang FE in a former life. The axle ratio swap will definitely give improved mileage, though somewhere on the order of perhaps half a mpg. May or may not be worth the investment but certainly if you can get it for cheap and DIY
Aluminum driveshaft will give better FE from weight and reduced inertia - drilling out a flywheel will have the same effect. I cannot think of any real world situation where an Al driveshaft will hurt.
IIRC the 7.5 and 8.5 refer to the diff gear size, in which case I'd recommend you stick with the smaller (lighter) one. The bigger one can handle more torque if you're doing burnouts and the like which I assume you're not. Besides being lighter, less fluidic losses.
Don't have an answer to your last questions. The "K" factor of your torque converter is at fault here but I don't think you can change that easily.
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