If you go with the 440 and use headers expect there to be an extremely high frig factor. The best fitting out of the box are the schumaker tri's which are not particularly suited for a high horse, high rpm motor and I believe cost around 600. TII are said to be well fitted but I have known guys that did a lot of swearing during their install, I believe they are around 900. If you are not going to be running street slicks, expect to have to figure on getting the fattest tires as possible up under the rear along with other suspension upgrades. If you are looking for eye candy go for the big block, if you are wanting sleeper do a 360 to 408 stroker and pay a hell of a lot less for headers and break parts less often. Parts limitations on the magnum are on the intake as you can use LA valve covers on the magnum. Also if you want to use greater than a 525 lift cam on the magnum, you will have to machine the valve guides down to provide enough clearance. A flat tappet cam can be installed in the magnum but you must use AMC lifters, and will have to measure very carefully to get the correct length pushrod especially if the block or heads have been milled or use a head gasket that is a different compressed thickness than the factory one.
Unless you use the original serpentine belt set up, you are looking at buying a non-serpentine belt balancer($122.00) and using the original flexplate or a B and M neutral balance flexplate. I did a big block duster and it was great fun but I would highly recommend that the car have power steering and the biggest torsion bar you can get. My current project is a 70 Dart 360 magnum.
