I just noticed this thread and thought I'd chime in. I have a T5 in my '66 behind a mild 5.0 roller cam engine.
You are very wise to think about this. A lot of people focus on the overdrive when doing a T5 swap and don't give any thought to the other end of the equation. First gear in a lot of T5's is pretty low and you run the risk of rendering first gear pretty much redundant if you go too low (numerically high) in the diff. I do think the Z's have a 2.95 first gear, which is what I have in mine - I got it changed from the 3.35 my transmission had originally for this very reason.
Tyre diameter is the other variable to consider and one of the calculators Mark pointed you to is a good idea.
I have 3.25's in my diff with 17" wheels and 225/45/R17 tyres. I find this a pretty good set up with good acceleration through the gears, its happy a bit over 80 in fifth on the flat and cruises at 100 in fifth at about 1,900 rpm (0.68 overdrive). Depending on your tyre size I don't think I'd go much lower in the diff unless you want to routinely change into second after about 2 metres, or you want to routinely wring the car's neck.
I wouldn't bother with discs on the back. The fronts do most of the braking.
If your car was originally an auto its probably got 2.80's in the diff but you can determine what it is easily. enough. Jack the car up at the rear, mark the pinion or tailshaft on the bottom, then put your phone under it on video record. Turn one of the wheels two full turns, then see how many rotations your mark on the pinion made - you'll have to guesstimate fractions of a turn but if its three and just a little bit it probably confirms your 3.10 suspicion, but I think 2.80 or 3.0 is more likely. Its unlikely but if you turn one wheel and the other one also rotates in the same direction (don't worry about it going the other way) you have an LSD of some sort and only one rotation of the wheel is needed to give you the diff ratio.
If this suggests you have 3.0 or 3.10 I reckon just put the trans in and see how you like it before spending the money on a different ratio, particularly if you have smaller tyres than mine. You can always change it later.