M21's are hard to sell unless its for a resto job. The M20 will work better with more highway friendly axles like 3.73s 3.42s and 3.31; and the M20 shares the same 1st, 2nd, 3rd gears and reverse geartrain. The difference is in the cluster and input (maindrive) of the 20/21.
The M22 does NOT have spur gears, however the only common gears it shares with the 20/21 family is the mainshaft reverse gear and the splined idler gear. All the other gears are M22 specific
Things buyers want to know:
Input Splines / Output Splines?
damage to case?
new wear parts?
7/8" or 1" countershaft?
Whats it work best with?
If I had a non-numbers matching M21; I would rebuild it as an M20 because it fits a wider range of cars and its probably easier to sell