I had this problem with an m46 once. I undid the two halves of the overdrive and it came straight apart.
Obvriously means a bit more clean up and reassembly before you can put the iverdrive onto something else.
Volvo's recomended procedure for an m46 is to lift the car so the wheels are off the ground, run the engine and put it into overdrive, then shut the engine straight off. This avoids the overdrive and gearbox sticking together.
No idea if this is the same procedure for an m41, though you are past the point where it would be useful in anycase.