Ima guessing it came to you without oil and an open cover?
If there is any trace in there of gear-oil, then there is a good chance that the cluster and pin may be ok. In which case, there is also a good chance it is salvageable. Not cheap, mind you, but at your buy-in, you should still come out ok.
I'm not keen on the ods behind performance engines anymore, but on low-rpm engines, they are a reasonable option.
The not turning, if the cluster is ok, is very likely a bearing, or both, piled up with rust.
If it was mine, I would pull it apart. On od boxes the cluster pin has to be popped out,towards the rear, as soon as the extension housing is unbolted.Just rotate the housing somewhere near a half a turn, so the pin can get by. That front "frostplug" will fall out after you bop it a time or two with a big punch.They can be reused if you get lucky.
Once the pin is out and the cluster is dropped into the bottom, the rest of the rebuild is the same as non-od boxes.
As for the rust, I have used both methods in the past, that is; wire wheeling, and media blasting. The clutch cones must be polished up on a lathe, with a quick touch of 120 cloth-tape. This will get the brass to bite again. All the bearing surfaces can similarly be touched up.All the C.I. parts can be sand-blasted, just be sure to clean out all the threaded holes. The extension housing needs special scrutiny. There is a little metal baffle in there under the vent that will need to come out for blasting, as well as the vent itself, as it is an excellent sand magnet. Just under that is an oil pocket, and oil-return slot. This area also traps sand.
After the blasting, I make a trip to the car-wash.
With new bearings, a gasket/seal kit, a small parts kit, and a rear bushing, you will be into it for less than $300. With any luck the synchronizers will be good, and you will have a fine tranny.
Now here's a nugget; treat the o/d gear nicely. That is the weakest piece in the box. I have ripped the teeth of two of them. One, with my HO367, and not even at full power. She's never been dyno'd, but similar builds pull about 440ftlbs at about 4400. And the 2nd with my 73-LC318; with nothing more than headers and a TQ-4bbl.
So, now, Good luck, and we are here to help.