A couple of other thoughts.

Somewhere on this site there is pic of a R129 with a BMW turboed 6.

Motor swaps must accomplish something that the original motor can't or would be very expensive. Sticking with a motor from the original manufacturer helps as there is some overlap in parts.

This can sometimes be skirted, look at some Daimler / Chrysler stuff. A seemingly simple swap would be Charger / Challenger / 300 bits in a MB that has a electronic shift 722.6 / NAG1 5 speed trans. This way the shifter, drive shaft, trans mounts more or less fall into place. The electronics would take a bit of work but, this could be overcome by making the HVAC manual and using the Chrysler ABS / trans control.