I think the m21 is a close ratio with a 2.20 first gear. You have to have at least a 3.70 to get the car moving.

I am not sure how high you can go and still have enough final gear to get the car going. The wide ratio suggest no less the 3.31 and my 3.36 with my m20 is high enough. I would not want to go higher or it starts making it hard to pull out. I bet you may need at least 3.73. To be happy.

Here is some reading on it. http://www.5speeds.com/ratios.html