Wrong Tranny Installed

Vehicle:
A 1991 Chevy conversion van G20 with a 5.7L engine, A/C, power steering and an automatic transmission.
Problem:
Transmission failed while on vacation. Mechanic convinced me to change from a 4 speed overdrive tranny (which he said was the same tranny installed in Chevy S-10 truck) to a 3 speed tranny which I believe is a turbo 350. He said it would pull my travel trailer better and I would love it. Well, I don't.
Question:
Does anyone know what the original tranny was? I would like to get the van back to the original shape before I was innocently accosted by this motorcycle mechanic approx 4 years ago. I can't remember the mechanic who worked on the van. It was in some little po-dunk town named Arlington, Oregon.