I thought I replied to this post before, but I don't see it now, so maybe it didn't go through.

Having reverse go out seems to be a common weak point of M-B transmissions. In my case ('91 420SEL), reverse started taking longer and longer to engage (seconds), until it wouldn't engage at all. Problem was with the B3 reverse clutch. It turns out that the transmission in my car (622.3 tranny, if I am remembering the number right) uses a reverse clutch, which has no external adjustment, and requires opening up of the tranny to repair. Mine went out at 165,000 miles or so. Repair cost about $1300, and the car now has 182,000 miles on it.

But, older 622-type trannies (622.0, 622.1, 622.2) have an externally-adjustable reverse band. I am guessing that a late 70's car would have one of the above in it. If I remember right, the adjustment is on the passenger side of the tranny housing, and is reachable from under the car. It basically looks like a threaded stud, with a locknut holding it in position. Some transmissions have a screwdriver slot already in the end of the stud, and so you just have to undo the lock nut to be able to adjust it. But I seem to remember that some cars don't have the screwdriver slot.

I can't tell you what the adjustment procedure itself is - but I'm sure if you search the forum you will find mutiple posts (try "reverse band adjustment", or variations of that). (It is something on the order of "tighten the adjustment screw/stud to X ft-lbs of torque, and then back off Y turns.") This may or may not solve your problem, but it is at least a low-cost thing to investigate and try. The adjustment IS supposed to help with slow engagement of reverse.