The 1973 Charger was a high production year for Chargers. Since you have a 400 I am guessing that it is a SE Brogham, which was the best seller. They are really good cars that got a bad rap from muscle car guys because they had some luxury in them. Interiors are really comfortable, and 1973 was the first year for the isolators under the front frame stubs to reduce vibration. I built the 400 out of my 73 Charger and it was a monster. I stayed with stock heads (906) and had the engine punched out 30 over, K B pistons, and Eagle rods were used. Huges sold me a great cam package with valve springs, and a Thunder series 800 CFM Eddlebrock carb topped it off with an RPM manifold underneath. I think total machine, and parts cost was somewhere around 4000.00. Sounds like a lot, but the power that the 400 with pistons the size of coffee cans was great. It was fun stuff. A modern Hemi conversion (5.7) would be pricey even if the engine comes from a donor and is cheap. You would most likely go with a modern trans, or get the swap kit for the original 727. Lots of tricky stuff, but more and more of these conversions are popping up.