I just purchased a 70 stage 1 4 speed car without a/c- supposed to have 3.64 posi but rearend missing along with original Muncie trans- my sloan paperwork says M20 , shouldn't it be an M21 close ratio? or were both trans available? seems like the M20 would be a very steep first gear with a 3.64. any help would be great as im just learning the buick thing. thanks