Well, I am looking at doing the same with a 5.7 hemi as opposed to a 5.2/5.9, but you already have the motor, so...

I don't see any issues with bolting the motor up, I'm pretty sure Schumacher sells mounts. I don't know about the 518 going in easy, I think at the very least you'll need some major trans x member surgery. And don't forget on the later Bs you have that extra cross member for the iso suspension, which might hamper things even more.

Mopar Action and MP teamed up to put out a complete FI swap kit for the magnum, and did a full write up across several issues. I would take a look at that and maybe get a few back copies. The project was called the "Savvy Savoy." I don't recall what they used for the trans.

In my opinion, four wheel discs are a waste. Most of the braking is done by the front rotors and even with 10 inch rear drum, if they are in good shape, you should be fine. I would recommend swapping to the newer manual master and upgrading to 11.75 unicast rotors (or rotorpros).

To lower the car, crank down the torsion bars a bit and get some de-arched leafs for the rear. Do it the right way.