Do you want to do this for better fuel use or for top speed?

You would have to use a LOT of fuel - even at euro prices to make any swap work.

I spent many happy hours in a US 72 450 -- it was a great car. I loved the looks with the short bumpers, and it was simple - manual AC / windows. In fact in many ways nicer to drive than my 560SL.

The early 4.5 motor W/O cats had almost the same specs as the 5.6 motor of the late 80's.

The lighter weight of the early cars especially at the front plus the 3 speed made them fun to drive in the hills of Pennsylvania - the 560SL is always down shifting.

Now I would think top speed driving would be something else -- but I do think it was still only a 3 speed in Europe in 1972 -- correct?