Poor & pathetic notation. This statement is misleading but true. Read more below.
(Insignificant, amounts that total squat.)
My favorite words above, "I can not tell you."
An insignificant amount.
You would NEVER EVER know the difference when driving it.
You will NEVER EVER know the difference in your wallet thickness when paying for gas.
The drag strip will show the difference more so in driver error.
The dyno is the only place where you can (sadly and pathetically) point the finger and claim (like a brat teenager) that your right.
GREENMACHINE
The 8-3/4 rear can be swapped in easy.
You will need to find an "A" body 8-3/4. The expense of cutting down another rear is crazy and stupid to do unless your doing a custom or drag racing.
The rear will also need the normal 3 inch U bolts and shock plates. The 7-1/4 uses a odd smaller 2-1/2.
The big expense after the rear end purchase is shortening the driveshaft to fit.
(Or fixing the worn out brakes.)
The 8-3/4's nose, where the driveshaft bolts to is slightly more forward or longer. The driveshaft needs to be cut down to fit.
As mentioned before, a 8-1/4 is MUCH cheaper and still very strong. But not available with the small wheel pattern.
Excellent gas mileage? Look for a 2.76 ratio, about the most common ratio there is. A 3.23 & 3.55 offer somewhat of street performance. But they drive up the RPM's and will consume more fuel.
I did indeed notice your engine. Can you post up some specs on the engine? I would like to specifically know about your cam specs. The duration @ .050 would be a BIG/HUGE help in suggesting a rear gear ratio.
I also get being 16. You have a lack of funds to do things much less do them twice! Frack that!
(Edited, added a ",".