A lot of these transmissions are built for circletrack or drag cars...quite often they thin down the reverse gear for less weight. This is fine for racing because you only use reverse to get it out of the trailer and to jockey around in the pits. It is based on the T10. You can contact Bubba at G Force South if you want parts and service. Im not being mean...its Bubba, I think his real name is Mike though.
Its not your fault; but a lot of people think you can just stuff any manual trans in any car and it'll work. You REALLY need to figure out what ratios your customized racing trans has before you invest in using it, especially on a shoestring budget. If it was built for a bullring track and a 5.14:1 rear gear; its going to be an absolute bear to use it in a car with 3.42s in the back. Your M21 has a 2.20 1st gear and really needs a 3.73 - 4.10 rear axle to work effectively. You could tear open the M21 and swap the input and cluster for an M20 input and cluster...this would give you an M20 and you could use 3.23s to 3.73s or more in the rear end