You can put a spacer in between the top mount so that a 7" diff fits a 7.5" beam. For the driveshafts it depends on what kind of diffs they are. There are 7.5" diffs with either 100 or 108 mm flanges. And for the 7" diffs there are diffs with 100 mm flanges and diffs without flanges, where the shaft is stuck inside the diff.

What ratio is best depends on what you want from the car. As standard I think an XR4i had 3.62:1, which is probably what I would choose too. But on my 2.9 24v I have a 3.36:1 as the engine has more power/torque to pull the longer ratio. This gives it a higher top speed and also makes it better on fuel. But if you only care about acceleration a shorter diff is better.