One of our friends/customers has a '65 GTO he drag races. In the earlier days, he used the Muncie. By the time power levels got beyond 500 HP and 500 lb. ft., the "little" Muncies (M-20 & 21) were proving to be not up to it. Tried an M-22 (large output shaft, TH-400 "yoke") for a while. They too, cracked the cases. Liberty gears and a steel mid-plate helped, but never got a whole season out of it. He runs a Jerico now. But this is a fairly heavy car with a high-torque Pontiac in it. It 60'-s in the 1.4s and goes 10.30s right around 130 consisitently. That's asking a LOT from a "stock" transmission.
Many of the T/A crowd use the Tremacs. The Kiesler "kits" are very popular. For a "drag" application, not the better choice. For a FUN street car, the 900 should take the power.
Another 4-speed option that can take as much as the Muncies is the Borg Warner ST-10. I like the older iron-case version better for drags. But a good, tough trans.
NOTE: My knowledge and experience with the Muncies and the "partculars" is strictly from the '70s and early '80s when they were still plentiful in cars. I can't "swear" to all the correct designations, just what we were calling them "back in the day". Best recollection follows:
Earlier versions were around, but rare. These are the "popular" ones beginning around '63 (I think)... I've seen a few from Bonnys and Catalinas with the "long tail". Odd-looking...
M-20 "wide ratio", usually behind bigger engines in heavier cars with "normal" gearing 2.56:1 1st (?)
M-21 "close ratio", usually in cars with high-powered engines and "stiffer" gearing. 2.20:1 1st
M-22 came in both configurations ("close" and "wide" ratios), upgraded gear quality and larger output shaft. Later muscle cars.
M-37 "Super Muncie" or "rock crusher" Several ratios available, large input shaft (1 1/4, like "Hemi" 4-speed and "big" Top Loader), spur-cut 1st gear (increased load "shock" capability, makes a horrid "whine", thus the nickname) and large output shaft. The only cars I recall them in were the ZL-1s. LOTS sold over the counter for racing in the '60s. If someone reading this has more specific detail on this one, I'd really like to know more.
Most of my Muncie work was in Pontiacs. Some of the designations, gear ratios, etc. are different with the Chevy versions, but for the most part, they're "the same", as are the Buick and Olds. I recall Pontiacs have the speedo drive on the left and at least some Chevys, they're on the right.
There's an Italian-made version of the M-22 available "new" today. They call it "Super Muncie". Reports are favorable, but I have no "hands on" with it. A search would turn up a vendor.
For a "road car", I prefer the "fine" spline. 26, I think. For a drag car, the "course" (11) has greater "shear" strength.