Still alive here, got back in town. Then I started to play with a few problems with my 280SE Coup. took the plugs out, went to put them back in, I had a stripped plug! I was so mad!
Anyway, You will have no problem with a 280 S transmission I assume it is a 280 S transmission for an M130 engine, not the late 70's M110 engine. I'm sure you will do this, but just in case you don't think of it, you need to replace the throw out bearing and change out the clutch disk.
I paid $350 dollars for a 3.23 rear from a 280SE4.5 that was wrecked with 45,000 miles on it. That was compleat with brake calipers in all. There are many places over here that have them, even with shipping you might spend less. I am parting out a 220SE that hase a 3.23 in it as well, but I don't know when I'll I'll get to it, (most people want interior stuff). I would see how the transmission works out, keep looking. Let me know how it goes.