I would prefer not to use an M5 tranny as it is bigger (I am sure?) than the Getrag 265, I have the bellhousing and transmission - sourced from a 635CSi manual car. So I have to get an adapter plate - no much of an issue and have someone who can do this. So the main issue is what clutch to use - what would be the best in your opinion?
The Getrag S6S 420G is indeed larger than the 265, I have had them next to eachother before when I was considering the swap.
With an adaptor plate, I'd be rather concerned about its thickness. To maintain enough clamping force, there must be sufficient thread distance, which means a thick adapter plate, since you don't want to go drilling in the engine block for bolt holes. A thick adapter plate will make the 265 a difficult fit, but that will be an ongoing problem, given the size of the S62 in the tiny E9 engine bay.
In my current swap, there is less than an inch of clearance in the trans tunnel (at the bellhousing) and about a half-inch of clearance between the fan and the radiator.
Speaking of the radiator, you will probably have to get a custom radiator, since I am not sure that the E39 M5 rad will fit, and given their propensity for blowing their plastic necks, it would probably be best just to get a Zionville one to start with.
Even though it is a 265, I would still be concerned with the transmission's capability to handle the motor. I'd have it rebuilt, probably by Metric Mechanic, unless someone else has a preferred manual trans bullet proofing solution.
As for the clutch, most of the FI guys use the UUC clutches, with great results. Like I said though, I have not researched this specific topic much, so some further sleuthing on your part would be advised, unless someone else has done the research.
