Not a conversion for the faint of heart...I own both types of W124 and they could be no more different. There's a 66 page MBZ publication that outlines the differences; we're talking: engine, tranny, rear diff, entire suspension (it even has an extra brace between the front lower control arms), wheels, tires, brakes, all four fenders, both bumpers, entire lower body; I could go on.
Seriously, if you've got the wherewithal to potentially upgrade a 300E, why not pop for a 500E? If you can't get there, a 400E/E420 is a helluva car, and with lower gears would be scary fast.
Just my humble opinion...best of luck with your decision
Best regards, Michael