Comfortable as it can. You would be running 3600 rpms @ 60 with the 3.36 and about 3800 @ 60 with the 3.55 with stock wheel and tires. But those gears would still suck pulling out with a 2.20 fiirst gear. M21 needs a 3.9 rear gear or you will be disappointed on take off from first.

Muncie 4 spd m 20 or m 21 was not meant to be efficient and if you want good take off, low rpms above 60 mph, you need to look at a 5 spd with a steeper first gear so you can use a 3.06 rear.

I would find a m20 where the first gear is a 2.52 then you would be safe with a 3.36 rear gear. You should be able to find one for about the price as the m21. 3.36 with a m20 would be about as efficient as you could go with a 4 spd Muncie. Unless you go for the auto gear m22z with the 2.9 first gear. Then you would be safe with the 3.06 rear.

You have to really think about the first gear take off so you don't burn clutches up just as important as the final drive rpm output. No need to be efficient on the top end if you have trouble getting to it.