Bore size versus fuel mileage. 273 versus 5.2 magnum

The 273 with the smallest bore will have an efficiency advantage by way of reduced surface area of the piston and chamber. That's less area for combustion energy to soak into the block and head. But it's fairly minor in the greater scheme of things. The low hanging fruit is going to be cylinder pressure. If you want anything resembling fuel mileage, pick a cam that closes the intake valve before 60 degrees ABDC. That's going to mean a mild or "RV" type cam. No rumpy-rump idle.

If it were a choice between a relatively antique 273 or a readily available 5.2 magnum, the 5.2 would be my choice every time. I've never seen a 273 with multiport sequential EFI, like every 5.2 has.