If you can find the 6 speed, I believe it will just bolt onto any M60, no big deal. The question is finding one, but any good dismantler should be able to get the parts with relative ease.
M50 is even easier - Getrags with the correct bellhousings pop up on eBay all the time for ~ $500...I never see them get sold.
You'd want to replace all wear parts with new ones, perhaps even do the seals on the trans if applicable/warranted.
Frankly, it seems like a pain in the ass and wallet unless you want the experience - just sell the car and find a manual.