I may be wrong but I think you need to use 77 up manifolds and header pipes to make the change. There is a '79 450 being parted out on another forum.

http://www.benzworld.org/forums/r-c107-sl-slc-class/1456723-parting-out-79-450sl.html

The owner is also a member here. You can PM him and see what he has.