My personal view is that the best way to enjoy a car is to keep it original. If I had the money to buy an M body as a 2nd car, I would want it to have the old reliable 318 cubic inch V8 with a factory-spec carburetor, to get the true experience of the car. If you really want a 3rd gen Hemi, which is a great engine, you can get a used Charger at a decent price and not have to worry about swapping engines.
If you do the engine swap on the Fifth Avenue, it's a lot of work, time and money and even if all goes well, you have a car that might have less collector value than before. If it really needs a new engine, I'd rather put in another 318 from that era.