new transmission question

[bkqb57]

I'm about to order a new tranny for my truck. It's a '90 4x4 with a 350 and a NP241? transfer case. I am not really sure which tranny I have(the pics in chiltons manual are differentbetween models on different pages-rpo code says its a 5-speed getrag), but it has an aluminum case. What transmissions can I use that will be an exact fit(length, wiring, direct bolt up to trns.case). Can i put in a heavier transmission from a 3/4 or 1 ton(this one only lasted 15k since i got it). where is the best source als? Ive never done this before and I would appreciate any help.

------------------
90 gmc4x4