Phil says: The main difference between M40 and M41 gearboxes is a completely different output shaft. It is much simpler (and possibly less expensive) to replace the M40 than it is to attempt to turn it into an M41.

The M41 was built into the '75 model year and is not difficult to find in salvage yards. 1800, 140 and early 200-series cars are potential donors. The 164 and the occasional 1800E were equipped with the M410, which looks similar but will not bolt to your bellhousing -- if the trans attaches to the bellhousing with four bolts from the rear, has an OD and the donor car has a motor that is not mounted at a slant, you've found an M41.