Transmission Question

I have an '89 Firebird formula 350. It has a "rebuilt" transmission in it, which has seemed to stop working (only reverse works). So basically I need a new one. I was wondering how difficult it would be (price wise) to go from AT to MT, and is it ever worth it? Any suggestions on what tranny to buy are welcome. Thanks in advance.

Quite pricey and a lot of work.