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There are M4 gearboxes with overdrive as they were an option on the early 122S, but they are as rare as rocking horse shit. The problem is that you can fit most of the internals of an M41 into a M4 case, but as I seem to recall from trying it once you can't make the gearshift work because the change mechanism is drastically different and you cant fit the M40/41 gearbox top onto an M4 case.
I think it would be easier to plug the holes in the B16 belhousing and drill and tap it to accept an M41 gearbox. The internals into the M4 case is a no no.
Incidentally the gearbox splines are the same. A B18 clutch will bolt onto a B16 flywheel, ideally you need to skim the flywheel flat first though.
Pete
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