Boy Kent, that was a mouthful you just typed....
It ain't actually that bad...but hopefully you have the whole '74 parts car.

Yes, the transmission, bellhousing, driveshaft...all that should work fine. You will also want to save the petal box with all the petals, the flywheel, clutch cable, clutch, etc..... And the

Not sure about the O/D switch in a '74, but in a '72 it would be a stalk opposite the turn signal switch. You might need a different cowling around the steering column with a hole for that new stalk...or drill a hole...or the one from the '74 might work. Don't remember for sure about that. If you aren't worried about the original look, a toggle switch could be made to work fine too.
You should expect to do some wiring to get the O/D to work properly.

There also might be a difference in the flywheel...thats for someone else to answer... But namely, it might be held on with a different number of bolts. I bought a car once that had been owned and converted by a machinest, and he filled the holes in the flywheel, and re-drilled it for the different blt pattern. Worked fine, but I wouldn't recommend trying that at home! Outside of that though, its pretty straight forward, but it is a lot of bolt turning. Well worth it though.

Of course, the rear end ratios are also different between the M410 and the auto, and the speedometer as well I would expect. To change that or not depends on your preferences and ambition....but again, if you have the whole parts car, you would be wise to save the rear as well...

If the '74 gearbox was quiet when you drove it, its probably OK. They get to whining real bad though....but if it seemed OK, and the overdrive felt solid, don't mess with it. You could do some seals I suppose....but don't fix that which ain't broke!

Good luck!
-Matt
72 164e (with a M410 conversion)
and others.