One example could be fitting 302C heads on 351C and getting a compression boost resulting in better power delivery, few more miles an hour in each gear.
Another would be the later bigger (by 2mm

170 ci head on a 250ci Falcon pre cross flow stuff also supposedly raised performance.
Can this be done with EJ components like an EJ16 head on an EJ18 or 20 , or the other way around like EJ20 /22 SOHC heads open up an EJ16/18?
Manifolds and trottle bodies?