I'm trying to remove the overdrive from a junk M 410. My manual for an M 41 makes it seem so simple...remove the nuts holding the OD to the intermediate flange, and pull the OD off. Not so on this one! A careful use of a hammer and drift ( and several hard whacks too!) haven't helped a bit. I've tried removing the bolts from the flange to the main case....that's no better. What am I doing wrong?

If you can't remove the tranny from the car first then try moving the crank back and forth by way of the lower pully on the front of the engine, and doing the same with the OD output flange. Then gently pry between the tranny and OD casings. You might be able to carefully tap a chisel or screwdriver between them to get started.
If the OD was left on when the car was last turned off it could be a bigger problem. In that case you may be able to put some power to the switch or wiring to turn it off (listen for the click sound from the solinoid), which would release the tension between the units.

Thanks. This tranny is out of the car, and has been partially dissassembled by someone else in years past. I've always hesitated prying cases apart because of potential damage to the sealing surfaces, but I'm just about to that point.

If you keep rotating your pry spot you'll work it out uniformly, kinda like walking it off. Then for the "scars" on the sealing surfaces you can smooth them out with some crocus cloth or very fine emery cloth before resealing.
Phil S. from VClassics told me that me that a tranny and an OD are kind of wed. But it seems it would work with the same kind of tranny. I'm not too sure about that.

I finally got it...I think it may have actually been a hydraulic seal between teh tranny output shaft and the OD connected. Slow ( overnight) wedging started it moving, and a slow pry from there finally got it pulled free.

Thanks for everyone's suggestions!