A 4 or 5 speed manual gearbox from an SLC V8 would be your best bet "IF" you can find one as it was a very rare option.
They were never marketed Andy
The porsche, who's model name I can remember as im not a porsche man, around late 1980's era, front engined V8 with the gearbox at the rear uses the same autoboxes as found in the 126 V8's, and they also had a manual option, maybe that would fit,,,, if you can find one!
They are listed on the EPC as a special installation on the SLC and SL 107, and were available in a 4 and 5 speed option.
I think that you will find only on the small engines,,not the V8
I know looked on my copy too, and its strange that no gearbox parts are listed either. I like you always thought there had been no manual v8 MB, it took me a lot of googling to find those two for sale, I nearly gave up.