Search the archives and the photo galleries- there's been a lot of discussion about this. You'll have to make up something to support the rear of the transmission- note the heavy angle iron in the below picture. The M41 will have a lot more weight cantilevered off the back, but with heavy enough supports it should be OK. Overall the wisdom seems to be that if you want a manual trans car it's better and easier to restore a manual trans car than to convert from auto. My 122 had been previously converted from auto to an M40, so I'm putting in an M41 this spring. Another thing to note is that because of the bigger tunnel for the auto trans the space for you accelerator pedal foot is a little tight when you put in a manual, but you get used to it.
Tom
NYC