The original T-5 trans in a 91 S-10 would be non-world class and have the GM bolt pattern. To mate to a 4.3 engine you could use the bell housing from a 92 or earlier GM car or truck with the small block chevy engine (or a 4.3 V6) and a 3 or 4 speed Saginaw, Muncie 4 speed, or T5 trans. Carefull on an Astro bell. I though they all had GM pattern T5s but just saw pictures of one supposedly came from the factory with a Ford pattern T5 trans, that bell would not work in this case. Also no go on a Camaro and Firebird V8 T5 bell, they tip the trans toward the driver, not what you want for an S-10.
How long will a six cylinder NWC T5 last behind a 4.3?? How are you going to drive that new 4.3, and what shape is the trans in before the engine swap?
Piper106