You will have to either swap out the beam or cut the diff casing of the 2WD diff where it mounts to the standard 4x4 beam to get it to fit, the 2WD diff is wider. not a massively hard job, there's a few threads up about it.
Supra diff conversion is expensive, especially if your stdandard 7" diff isn't broken, converting it to a 2wd diff would be a lot cheaper IMO. Also the rear driveshaft couplings/diff output shafts between the 7-1/2" & 7" diffs are different.

in addition, i also read...somewhere, i think it was the escos forum that there is basically no difference between a 7" diff and a 7-1/2" diff in terms of strength, how true this is I don't know.

As for the Supra diff conversions, I'd speak directly to MK or Turbosystems for this.

Defiinately don't take this as a negative response, I am more of a 'break and uprate' kind of guy, I would definately consider this IF i had a spare bunch of cash or kept smashing diffs.

p.s DON'T convert it back to 4x4 unless you get a proper strong box, the standard ones are shit and you'll regret it when it breaks.