Assuming its a cammy screamer (140bhp x flow) ,I would have thought you would need at least 4.1 rear gear,although if you are on 175/50s and intend to do some mileage, a 3.89 or even a 3.77 might be more sensible.
I have a tran-x slipper in the back of my mk1 tina on a 4.1 with 175/50's and its very low geared ,although my x flow is considerably less tuned than your one.
Standard shafts should be fine,I've never broken one,not even behind a turbo yb.
I also fitted a panhard rod to my rear end with decent rear shox and some de cambered single leaf springs, and it handles very nicely just make sure you get the nose angle of the diff about level.
puddy