They do. I have done 4-speed swaps on both M- and J-body cars using the F-body parts. Regarding the '80-'81 M-body coupes, it's literally a part-for-part swap since those were the only M-bodies built on the 108" wheelbase (they actually use F-body coupe doors and windshield). On the longer-WB cars, you need a new/rebuilt (lengthened) driveshaft. I thought about using a B/E-body trans on my Imperial but the shifter ends up in the wrong spot, although a '71-'74 B-body bench shifter would be pretty close.
I've got some in-process pictures of the swap on my '81 LeBaron if anyone wants to see 'em, although the car was never finished.
I'm pretty sure the 4-speed tunnel in my picture came from you, Dan. It's for my '82 Imperial.
