Lots of little things plus some big ones add up maybe?

A stock'ish 383 bottom end can carry 500 HP so OK there. Cylinder head flow matters. The G3 Hemi has more. Compression and chamber design matters, the Hemi has better. For MPG, transmission and torque curve matters. 6 or 8 gears vs 3 or 4. Also variable valve timing and variable intake to help low RPM performance. MDS also helps when "loafing". Also aerodynamics helps a lot. Also EFI which handles different gas blends well and allows close tuning for WOT and cruise.

A BB can make the power and with some of the tricks above get OK mileage. Mine does!

Still hard to get both the power and efficiency combined of a newer design engine.