How does the clutch feel. I’m not discounting a slipping clutch but thinking of what else could cause it to slip. How about a sticking clutch slave or release carrier sticking on the gearbox nose. Noise could be the release bearing working hard and getting hot.
In the overdrive the uni clutch would have to fail and the cone clutch wear to get slippage. I have known an overdrive with destroyed uni clutch to start slipping. The owner didn’t know that the uni clutch had gone because the car had always been like that!
If the box is coming out I’d get it and the overdrive checked out anyway