[Nick M3]

So, your biggest hurdle in this project is likely to be passing an OBDII scan. I don't know for a fact that this applies to MBs, but I know that OBDII BMWs in the '90s would not pass emissions with auto to manual conversions without convincing the chassis that it was supposed to be manual. (Not that hard, but the MB is a lot more complex and has a lot less support.)

If you can put historic tags on, or use an OBDI car, you bypass that problem.

I actually don't think that the physical manual conversion is going to be *that* difficult. It's just that you need access to a machinist who can make you a custom bellhousing. That's probably a better route than trying to source an extremely rare and probably expensive transmission. R129s were sold with three pedals from the factory, so you can source the pedal box and hydraulics. (It's worth noting that in the linked thread, people point out that it's actually highly unlikely that it was a 928 transmission used and speculate that it was probably a BMW unit.)

Hell, it's likely that the V8 and the V12 share bellhousings, so there might even be an off the shelf adapter out there.