GTiR gearbox (RA version) is non-FRONT lsd... the front diff is located inside the transfer case, not the gearbox bellhousing, thats the centre diff.
Alot of U13's had a front viscious LSD. All GTiR/U12/U13 AWD gearboxes have a viscous centre differential.
The diff ratio your talking about does not matter, you just have to make sure the transfer case (which houses the front diff) ratio mates the rear diff ratio. So, you can use a GTiR gearbox with your existing U13 transfer case bolted to the side and you'll be sweet, you do not need a U13 specific gearbox.
I am using a GTiR gearbox with a U12 SR20DET transfer case and a U13 viscous front diff unit.
If the clutch felt funny, maybe its the throw out bearing or clutch fork that are damaged, or clutch itself. The gearbox will not want to select 1st and 2nd (or any other gears for that matter) if the clutch is not properly disengaging. Don't jump to gearbox damage conclusions without properly looking at the possibility of something clutch related being the problem.