71 GS 350 4 speed, M20 or M21?

So, I've bought a 71 GS 350 4speed out of PA home and it has what I think are 3.08 rear gears. Taking off in 1st seems to take a lot of RPM to get rolling. Is there a sure fire way to know if it is an M20 or M21 without having to crawl under it? Assuming it is factory, I'm looking for an "A" or a "B" at the end of the production code.

Here's what I know so far, assuming the tach is a) accurate or b) at least consistent:

1st @2000 RPM is about 20+/-1 MPH on GPS
2nd @2000 RPM is about 30+/-1 MPH on GPS
3rd @2000 RPM is about 40+/-1 MPH on GPS
4th @2000 RPM is about 50+/-1 MPH on GPS

If it is an M20, I'd expect:
1st 19.9@2000
2nd 26.7@2000
3rd 34.4@2000
4th 50.2@2000

If it is an M21, I'd expect:
1st 22.8@2000
2nd 30.6@2000
3rd 39.3@2000
4th 50.2@2000

When I rev to 3000 and shift, it drops about 1000 RPM each gear. I will eventually crawl under it but I need to change something, either rear gears or put a tremec in it once I hit the lottery.

 

seems to me all buicks got m21s. Which would explain why its a bit slow in first. I'm almost sure of this..

 

71up came with the 26spline input 32 spline output m21 id imagine its still in there no buick 350 ive ever been around could hurt a muncie

 

So, what's the best way to get more enjoyment out of it with the least amount of dollars spent?

First, I want to swap to some 15" wheels and wheel well-filling tires which will only make it worse. Is an M20 or some other trans with a lower first gear an easy option?

Second, is there a cheap/plentiful 5 speed that is an easy swap? Uses the same clutch, bell housing and drive shaft?

 

M20 will provide better take off but have a sharper rpm drop due to having the gear ratio farther apart, the 21 was intended for more powerful engines that didnt require a steep 1st gear, so with out either having some mods done to your trans ie lower first which Library Trans out of MI can do , you'll just need more rear gear or power, I myself would just gear the car up to a 373 with a small block, Liberty can also put an overdrive 4th gear its not much like .85 or something there about

 

A Richmond T10 is a very nice unit and offers first gear ratios all the way up to a 2.91 I believe but I know for a fact they offer a 2.8x,the trans is a direct swap, all the 5speeds I know of will require different bellhousing and more than likely a hydraulic clutch

 

3.08 gear and a M21 is brutal. I swapped to a 3.73 but now it's a bit to much RPM for the highway.

 

I just swapped out a 2.73 to a 3.23. I had the 2.73 in from long ago as a highway axel. The 3.23 is a great compromise if you expect to do some highway driving. I turn about 2500 at 70 which is not too bad. And yes the 2.73 was particularly brutal.

 

I just found out my brother has a Richmond 5 speed with a shifter in his basement that he was going to put in his car and never got around to it.

http://www.jegs.com/i/Richmond-Gear/836/7021710A/10002/-1?parentProductId=747494

Everything I can find says this is a pretty straight forward swap for a 71-up Muncie. Worst thing looks like maybe having to work with the trans mount and/or cross member.

Anyone done anything like this and have any input?