It's fairly easy to add the 4th gear swith onto an M40 long shifter top. There is a thick cast ring on the inside of the cover in the correct place. I took the switch to a hardware store and tried nuts on it until I found one that fit. I can't recall the size but it was metric. Then I got a matching bolt and a slightly smaller drill bit. I drilled out the cover centered on the ring and ground slots in the bolt to make a $.50 tap. Then I carefully screwed the bolt into the hole to cut the threads. Then I was able to screw the switch right in (with plenty of that black sealant stuff that hardens like a rock) and it worked perfectly.

Swapping the tunnel isn't a task for the faint of heart - it involves chopping a pretty large hole in your car and your carpets won't fit afterwards either.