I've never seen anything specifically listing exactly which ones were offered, but I believe all versions (M20/M21/M22) were offered. From what I've seen, the M21 was most common. Starting in '69 (and a few 68 Nova SS396s) a special TH400 was offered as well.
Dcarr, M-20,1,2 were all available. Some of the literature designated that with certain rear gears only certain trannys were allowed. For example, if you got a 3.08 rear, you had to take a M20, wide ratio. But, to answer your specific question, all 3 WERE available in 68. Period.
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