IMO the M41 is a poor choice for a race car, the overdrive unit won't hold up to the abuse it will get. It would be better to get a limited slip diff and try to find a final drive ratio that will best utilize the power band of your 142 in the road courses you will typically be driving on. I believe the M40 from a 75 240 is the beefiest 4 speed available.

If you decide to use the M41 anyway:

Some M41's came with the smaller flange, if you can find one they are relatively easy to change. You will need to shorten the front half of your M40 driveline to fit. If your large diameter driveline came from a BW35 with the long tail piece it may be the right length for your M41, I have seen a range of opinion on that subject and never tried it myself.

The Tranny is mounted in the same place as the M40. I use a 164 motor mount instead of the smaller and weaker stock mount. For a race car it may be good to mount the engine with 164 mounts as well.
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Lee 75 244 (80k original miles) 79 242 65 220 project