Some considerations which may be helpfull.

1) I know of T9 swaps into Volvo 122/1800/140 using adapter plates from three different sources.

2) The T9 is one of the most widely used 5 speeds in the world, though it did not have wide use in the US.

3) It is the most popular trans for swaps into MGs, etc.

4) Anyone who has driven both a 4 plus overdrive and a 5 speed on a similar road where frequent shifting from 4th to 5th, back down, then back up, is necessary will know why a true 5 speed is much nicer to drive.

5) The T9 is lighter and has a higher torque/hp capacity.

6) There are less parasitic power losses with the T9.

7) There are parts for the M41 that are no longer available and others that are prohibitively expensive.

8) Parts for the T9 are readily available, as are rebuilt transmissions, in several versions.

9) The parts needed to do the swap are as follows:
a) adapter plate
b) clutch disc with splines to fit the T9
c) slider yoke for the end of the driveshaft

10) modifications necessary include:
a) have a driveline shop attach the slider yoke to the end of the driveshaft.
b) turn down the input shaft sleeve to fit the Volvo release bearing
c) modify the trans crossmember to fit the T9
d) modify the speedo cable
e) drill a couple of holes in the B18/B20 bellhousing

I hope this helps. If interested email me directly and I can send further details and photos.

John
V-performance.com