I was in England last year (I'm American) and picked up a copy of Mercedes Enthusiast. Have since found the magazine in town.

Anyway, in this one issue, a guy in England dropped a m117 into the 123 body coupe. Looked sweet in the pics. Not sure what they did for tranny and rear, but it's do-able. I image it would work in the 115 coupe as well. The 116 450's have less bulky exhaust manifolds, then the 280se 4.5, so if you go that route find a late 70's 450. But if you want to carb it, guess you'll have to get a US v8