I have both a automatic and a standard transmission that I'm willing to part with. The automatic is for a M130 engine. The standard I'm not sure about, might be for a 129 (250 engine) but should have the same gearing or very similar.
It sounds like you do have a 280 tranmission, the gearing for most of the 6 cylinder cars of that time where similar. So it most likly won't matter what you put in there. This would however be a good oprotunity to put something geared more to your liking.
The rear end change is a nice way to go. I wasn't sure at first but some friendly MB tech convinced me. The speedometer reads a little off but not much. I'm half deaf so the 2600 rpms or so, makes it much easier to have a conversation.