I have a rust free W107 450SLC parts car which I eventually may do some crazy project with, so I'm quite interested in your project

However looking down the engine compartment of my 500SLC, there ain't a lot of clearance anywhere....

I have been thinking about the M119 since it is based on the M117, but I doubt there is space for the bigger heads. The V12 might be less wide but I doubt it. To overcome the width problem it would probably be required to place the car in a jig and redo the structural parts of the front.

In your case, why not go for a 560 euro spec engine, this one will give you 300 HP. It will however not give you much more torque.

Looking forward to your findings

Karsten