I'm thinking he is concerned with the gear ratio compatibility. The 2.20 first gear M21 Muncie was available with 3.55's out back in a few different vehicles. I've driven this combo and find it works okay as long as you keep stock tire heights of around 26". If you go throwing 28" tires on it there is a noticeable difference in the amount of clutch slipping that needs to be done. The effective overall ratio with this is 7.81.
You get nearly the same overall first gear ratio with the 2.52 Muncie and 3.07 rear gears. I've also ran this setup in 2 different cars without complaint, easy to drive with 26" tires. I actually liked this a bit better because I still have good take off, it feels the same going up through the gears as the overall final ratios are extremely close, yet I had a better highway cruising rpm with the 3.07 final.
My favorite is a 3.55-ish final with the 2.52 Muncie. It's quicker than the common 3.73 and 2.20 setup and has a little better cruising rpm to boot.