If you are sure you have a m21 with a 2.20 first gear, you will need at least a 3.73 to get it moving and be happy with the result. I would not change a thing on your car until you ready to do it all.
When I got my car someone had switch the rear gear to a 2.73 it still had the m20 in it. The car was not drivable when I got it. So during the restoration I left the 2.73 in until I was able to find the correct 4 pinion posi and correct 3.36 gears.
During the two summers driving it, I had a very hard time pulling out of even the slightest hill. It was even hard getting it moving with people in the back. Not happy with it at all.
I finally found the correct posi and gears and had Jim from JD Race rebuild the complete rear end he rebuild the posi unit and added the 3.36 gear. I got those in spring 15 and had them in all last summer. They made a world of difference. It still is not a performance rear end taking off. I would probable like even going lower to maybe a 3.55 but I have to try to get a happy medium with my upper rpm's. In 4th @ 60 I'm turning around 2700-2800 rpm. With the 3.55 I would be around 3000 rpm. There are rear gear RPM calculators on the web to show you these differences.
Because I left the 2.73 gears in for the two summers. I am now lookinng at replacing my flywheel and clutch again because I burned it several times trying to get the car to move will the 2.73 are in.
So it may be better to just not change anything till you are ready to do it all. You have a great rear gear for you auto now. If you go higher then the 3.73 with that m21 such as 3.36, I don't think you will be apply using the posi anyway.. You be much happier playing with the posi with the 3.90 gear.
