3.73 gears are what most people put on a 5 speed. 4.10's are good for automatics as they are only four-speed. A T-lok is a limited slip differential unit that allows both tires to spin and not just one. The limited slip allows the tires to rotate at different speeds when making turns so that way your tires won't chirp or skip. When installing gears and/or a T-lok, you will also need an install kit. The kit comes with all the shims, bearings, inner axle bearings, gasket, and some other goodies for the mechanic. Other than that, you will also need 4 oz. of Ford friction modifier, and you might as well get a set of outer axle bearings, so when they install the stuff, they can replace those too as the axles have to be popped out anyway. Only use Ford friction modifier on Ford rear ends, don't use any other brand! Trust me on this one. Lastly, you are going to require the appropriate speedcal, which is a little gear that goes into your tranny to help correct the speedomter. 3.73 and 4.10 gears require the 28-tooth gear (I think). They don't make a gear specifically for the 4.10's so you will either have to have a buddy pace you, and reposition your speedo needle manually, or on the 99+ Mustangs, you can do the same, or get an electronic calibration unit. Don't know if the last option is available for auto's though.
On a side note, your odometer will be off, and will clock more miles than you actually travel, but stick to your maintenance schedule anyway, as the motor is working harder now. SO when the odomter says 3K miles, go go get that oil changed.
After you install the kit, have the fluid replaced after 500 miles in the rear end, and have them check everything out just to be on the safe side.
3.73's on a standard won't hurt your gas milage too bad depending on how much highway driving you do. 4.10's on a standard are going to cut into the milage.
3.73's on an auto are probably a waste of time, and you won't get the full benefit of the acceleration mod that you are doing. 4.10's are decent for our v6's.
GEFracing.com has the gears, and T-lok, speedo gear.
50retso.com has the install kit (you will need the expensive one).
Texasmustang.com has the electronic speedo recalibration chip (I think?)
Ford OEM T-lok is nice, but the cream of the crop is the Eaton Posi unit (or T-lok). It has a conical clutch system instead, is rebuildable, and one of the best units out there. Although, you are going to spend three times more for this unit than the OEM one.
Also, a lot of people convert to the 8.8 rear end, but you can get forged axles for your 7.5 rear and be every bit as strong as the 8.8. Just an FYI.