There were 2 different 5 speed gearboxes put in 140's (and also 129's). First there was the 722.5, this is a fairly rare transmission, pretty good tranny though. It was ONLY put into 140's and 129's with the 104 inline 6 engine. The 8's and 12's used the old 722.3.
Later on when the 722.6 came out, they used that tranny in the 129 and 140's.
I don't think it would be easy to swap back and forth, plus the ratios' would be screwed up.
I believe the rebuilt would be a better bet also, due especially to the warranty. I'd check to see what the warranty is on it exactly.

Gilly