A lot of good info in here. If its a GM gearset; its an M20. However, a lot of companies back in the day made replacement gearsets, and we have no clue if they knew what the ID grooves meant.
You can put it in 1st gear and turn the input ten times; if the output turns 22 times (2.20 1st gear) its an M21 or M22. If it turns 25 times (2.5 1st gear) its an M20. You got a reasonable deal, if it doesn't leak like a stuck pig. Make sure you adjust the linkage properly, and the shift stops as well (if there are any).
Don't lean on the shifter as said before, it makes the shift forks and synchro teeth wear faster/ unevenly.
You only really need a quart of trans fluid. I know they say fill it up; but that fill hole migrated a lot. The gearset needs a quart to live happy. I'd go to Napa and have them order a qt of Sta-Lube p/n SL24229. Its a non Synthetic GL4 rated 80w-90 (I think, might be 75w90). Its a bit thicker than the synthetics and its not as prone to leaking out of every crevice.