Franz's twin turbo engine is in a a 600 not a W108 if you look closer.
Plenty of room in a car that was designed to house the M100 engine originally
And to make a finer point,the 600 is car that was designed to be big and fast and anyone who has driven one will tell you that they drive just like a sports car . More power means it will go harder and still handle like a car 1/4 of it's size.

here is a W111 (same floor pan as W108) with a 6.9 to show that "yes" they do fit.
http://bringatrailer.com/2008/04/13/405-lb-ft-69l-transplant-1968-mercedes-coupe/

The single turbo engine runs just like a factory built car. It was built in 1978 at a cost of $50,000. And that didn't include the brand new 6.9 either!. it's been owned by some well known mercedes enthusiasts but it took Phil to get it restored and running again after it had been parked away for many years unused.