Re: m20 4 speed to a t5 5 speed conversion?
I would consider that if I were in your shoes. However:
I wouldn't run anything but a 2.95 first gear GM V8 T5. Any other stock T5 gear set else is a gearing mismatch for a healthy small block. Especially for 3.73s. So if you want it to drive like it should, do that. I prefer the 0.63 5th gear.
The front of the case expands slowly over thousands of miles. The countershaft bearing race hole in the case goes from a circle to an oval. While a Muncie pattern G-force case is readily available (they make it, and a Ford pattern case,) they will do the same thing in 150,000 miles.
The 88-92 F-body V8 trans. has cable speedo drive, a canted transmission mount, and longer total length. The 88-up has multi-surface blocker ring surfaces vs. the earlier design. It will take some work to fit it.
I could go on and on. But a fresh, proper operating condition GM V8 T5 would be an enjoyable driver. If you're going to pluck an S10 box with granny ratios, early synchro design, and VSS-only tail, then you may be disappointed in everything but the shifter position.
If this is a car you intend to keep, do it right the first time so you can enjoy it as much as possible. That may mean saving up for a new T56 Magnum, which is often a really good option, when it comes to finding a core, doing fab work, working with shifter position, etc. on the older production stuff.