any of the suby trannys will fit. you may only use 4 of the bolts instead of 6 depending on what combination you use, but it'll work. get a later svx tranny if possible because they came with filters instead of the earlier ones that had a cloggable screen in the oil line. if you get one that has a different gear ratio than the factory, make sure that you get a matching rear end for it. if you want to go manual, you just need the clutch/flywheel that came with it, the tranny crossmember support, driveshaft, rear end, and pedal box/associated cables. plus the welding for the new pedal box.
a new tranny and possibly matching rear end isn't hard, but the low reliability factor of the svx in general is something to take into consideration.