I've heard the flywheels are the same between the 18r and 2t, so i think it's possible to run the whole 18r flywheel/clutch setup, going any bigger and it wont fit in the bellhousing.
Other option i'm unsure of is if the 3t's used bigger clutches?
Not sure about splines, but if you go into repco/bursons they have a good table that lists all the diameters/splines for heaps of clutchs, and you'd be sure to find a bigger diameter with same spline as the t50.