With a used tranny you may get lucky and ot works fine and lasts a long time or you may get 200 miles out of it and your stuck with $800 yard ornament.
If you rebuild using a quality shop that uses quality parts (there are aftermarket sources for heavy duty clutches/bands) you know what you're getting plus a good shop will iaually garantee their work.
Ultimately it's your money and decision.
The only option for a different auto tranny would involve big $$$. A cuatim adapter would need to be fabbed, etc...