The modifications to fit the 119 4 engine were not as significant a change as for the 119 5. From what I have read the subframe was altered somewhat for the E500.
Here is a quote from a road&track article posted on Dean Albrech's site:
" The front axle crossmember and myriad underhood compartments are also modified to accept the big V-8, which weighs 70 lb. more than the 300's 6-cylinder. The heavy-duty battery is moved to the trunk, mostly for balance, which reaches an ideal 50/50 ratio with two people and 175 lb. of luggage aboard.
Because of the additional weight and because the 500E is designed for performance, it has sturdier and shorter springs and plastic buffers, and gas-pressurized shock absorbers with auxiliary internal damping springs fitted to suspension that includes modified MacPherson struts up front and Mercedes' distinctive 5-link setup at the rear. Wishbones and steering linkage, taken from the 500SL, have been reinforced to accommodate the increased weight - 265 lb. to the front and 254 lb. to the rear of the car."
Go to his site and also check out the article on the AMG Hammer. Similar but not as extreme modifications were done to the Hammer too.
I don't think the E400/E420 went through the same mods. Maybe it has some but maybe not as many mods. Your right it would be a good project if it could be done.