How about a little free thinking here? I suggest a 4.3LV6 installation! I finally let my wife drive mine, and her first question when she got home was "Is the speedometer correct on your car?" When I assured her it is, she commented that it must be hard to hold it down to 60... (which it is, it likes to run about 75 or 80... around 2150rpm - 60 is only 1750rpm) Since the car has very little wind noise and the engine is absolutely silent, it's hard to notice just how fast it's going. I must say that the combination of superior German body craftmanship and GM power makes for a terrific machine! You can view my swap at:
My car was also very, very inexpensive to put together! Under $1K total...
Go for it!
'82 Mercedes 300TD/4.3L V6
'77 Jag XJ6L
'74 Mercedes 280C