If you have either of the manuals, you'll want a new tranny. If you have the auto, it would work with the 350, but by the time you got through with addaptors, flexplates and torque convertors, you'd be dollars ahead with a 700-R4 or TH 350 that usually comes stock behind the GM V-8.
700-R$'s and TH-350's are very easy to bolt to an NP 231, Usually with only an input gear change.
Well I have an auto but I also have a 350 torque converter sitting around. Assuming I already have a running engine with exhaust and in take, what will the switch cost me?