Years ago when I was faced with the need to drop an M41 to be able deal with a non-working 4th gear lockout switch but did not have the facilities at that moment I took a different path temporarily to be sure I did not engage OD in reverse. What I did was add a buzzer between the OD relay (round three prong) and the reverse relay (also round three prong). Engaging OD provided + to the buzzer and engaging reverse provided ground to the buzzer. If both reverse and OD were engaged the buzzer would sound off.
It was fun driving it that way-being able to engage OD in the lower gears. Many British sports cars with the Laycock OD were wired to allow OD in 3rd and 4th -- some even in 2nd. For peace of mind I replaced the defective 4th gear switch at the first opportunity. -- Dave

Thanks for the heads up, I'm not totally sure I understand you. You're saying the M41 came in more than one configuration, early and late? And they are substantially different? I was planning on using the existing cover, with long shifter, that's on my M40 now on one 1 of the od trans that I have. I have both a J supposedly from a 140 and a D from a P1800. Both transmissions look alike but have not compared them to whats in the car. The patient in this case is a 67 estate.

I don't want to touch the sheet metal or mess up the carpet if I can help it.

When I put a fairly late model M41 in my PV, I used the 'long shifter' top from the PV's original M40 (1963). It had no hole for the OD switch, but inside the cover there was a cast in boss in the correct position.

I took my OD switch from the short shifter top to a hardware store and found a bolt with matching threads (I forget if it was standard or metric really), and bought that and a slightly smaller drill bit. I drilled the cover out, and then took an angle grinder and made a crude tap out of the bolt.

Some WD40 and wrenching back and forth later, and the cover was tapped for the switch.
--
'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)