It has been discussed here in the past. Mercedes never made a manual trans for the early V8's. Even if you could find a suitable box, you would be up for a custom adaptor plate, Flywheel and clucth combination grief, pedals, shortened or lengthened/re balanced tail shaft, custom box mount , possible gearbox ratio, diff ratio incompatibility, manual or hydraulic clutch mods and at the end of it, starter motor combinations the list goes on. Even if you can fabricate the bits, you could wind up with a vehicle that drives like a dog due to bad ratio combinations. Modification compliance is an issue here, if it is in the States, you'll have the Wallopers all over you like fleas on a dog for non compliance. Sorry to hit you with so many negatives.
Brian.
280SL Euro
500SEL Euro.