All things being equal (and they never are) an M21 will be marginally stronger than the M20 and we're talking percentage here. The farther 'away' from 1:1 you get, you lose some strength.
Total Gear Reduction is essentially the mechanical advantage of the geartrain to move the car (first gear times the axle ratio).
The reality is you will have a total gear reduction of '8.2' with the M21 and 3.73s (2.2 x 3.73) Thats a pretty soft 1st gear. Use the M20 and 3.73s (2.5 x 3.73) This will give you a TGR of 9.3. The vaunted M22 and 4.11s (2.2x4.11) was 9.04.
This is all assuming both available units are in good shape. I would also prefer a 1" countershaft over the 7/8" countershaft for strength and for parts availability.
Also note that if your car was originally 32 spline, a 27 spline setup (or vice versa) may cause shifter-to-console interference. AND pre 1969 units use a stud mounted linkage on the sidecover and reverse levers; 70 and later units use a bolt-on type. These do not mix and match well.
None of these problems are insurmountable as long as the unit is in good repair.