Easy stuff.

Find a used, low-mile 5.9 Magnum from a truck, maybe even from an 'R/T' if you're lucky, it should be bolted to the OD trans already. How easy is that? Grab everything you can from the donor and get hackin'. Edelbrock makes an RPM intake for that application. A Holley 750 vac secondary should be fine. The engine is out, so buy some MODERN headers for it, something that doesn't go under the front suspension (TTI?). Put a good exhaust system on it.

I dont know much more about these engines, but a cam upgrade should be cheap enough? I also know that they actually sound pretty nice (for what is basically just a new head on a very old design), and that even big Rams with them can turn heads. They probably make more power than the good 340's did.

I also know that they're dated technology, and common as hell... thus cheap. The bolt-ons should cost you as much or more than the engine/trans combo. You should be able to keep that big 72 Charger on the lighter side ov 3500lbs with a SB combo...