I did the switch on my 72 142 and learned a few lessons. The pedal switch was time consuming but not to hard. My 142 had a smaller diameter drive shaft than the m40 from my donor car (73 145). Origanally everyone just told me to switch the front end of the shaft. Obviously I couln't do that because my origanal driveline was to small. I could not switch out the flange on the rear axel to accept the larger drive shaft (I couldn't get the old one off at first, when I did the splinds did not match so I couldn't use the flange.). I ended up switching out the entire rear axle. What I learned about the axle switch was that I didn't need to remove the half shafts to remove the axle like the book I was using said (even though I did).
My only screw-up with the whole switch was that I didn't strip the donor car as much as I wish I would have (kicking myself now for not having some of those parts I left on the car before sending it to Island Recycling).

Good Luck