For a time, some time ago, I had a garage find '74 2002A. Got it for next to nothing, it was pretty beat but was easily made to run fine. Having very little invested, I flogged it a bit, to the end result that reverse finally "quit". No reverse, no way.
I looked about and figured that if it was broke, there was nothing to lose by tinkering. I removed the drive shaft and giubo, found that my 30 mm socket would only just get the output flange nut off, and voila, off came the flange. Next, the final housing, about six inches long, it's illustrated in the cutaway above. Inside that final housing is a piston (see the cutaway again, piston on top and inside final housing) that somehow controls reverse. The piston must slide freely; mine had a near-microscopic burr that only just barely hung up the piston. I used 400 grit wet sandpaper to clean up the piston; reassembled and all was well as long as I had the car thereafter.
YMMV, but if fluid and filter are found in good shape, that's where I'd go next. Note that not all 30 mm sockets can reach. You ought to have a deep thinwall socket, although mine was standard Craftsman issue and took some light tapping to seat on the flange nut.
If the trans is otherwise sluggish, suggest it will loosen up after some running, heat cycling, and fluid/filter replacement. We live in hope to hear of your success.
