To avoid scope creep, keep the car on the road, and give you more time to define longer term objectives, I suggest an unlisted option:

Tear down the 273 and re-ring it with some JC Whitless super seal chevy rings of the appropriate diameter, file fit to minimum allowable gap, and give it a dingle ball hone. Install a set of Aerohead rebuilt iron heads for $400. Adjust the shift linkage to eliminate the missing low gear capture, put it all back together, and drive the piss out of it for under $600.

If you really don't want to tear into it and have abplace to store the 273, then start calling salvage yards until you find one with a running, low mileage 5.2 magnum. Swap out the efi intake for a standard manifold, carb it, bolt it up to your 904 and drive the piss out of it for under $800.

Then you can decide later if you really want to continue working on it after daily driving a Gen 3 Challenger.