It might not be the OD unit at fault.
When I assembled my 1850 without the copper washer on the bottom bellhousing bolt the gear oil leaked out continuously. I still drove it all the way to TDCIR and back again, topping up the oil as I went. While this was a tremendously bad idea it did give me a good view of how oil level affects the overdrive.
With an over-full gearbox the engagement is almost a bit violent, and then as the level in the gearbox drops the engagement gets progressively lazier until it eventually refuses to engage.
Based on what you've said I'd wonder whether it could be the gearbox instead; it might well be the OD, but it's something to consider.