The gearbox is dead, and the economics of the situation make used gearboxes much cheaper than rebuilding a box that has ground itself up. The OD tranny will make a great addition to the wagon - without the special driveshaft you just have to have the current driveshaft shortened at a drivetrain shop (they do this sort of stuff for hot-rodders all the time). You might need to do some work on the crossmember as well. Make sure you utilize the 4th-gear switch on the M41 when you wire up the OD. If the M41 has the remote shifter top swap the one from your M40 and drill/tap the switch into it.