Biggest changes i have consistently seen always involve heads and cam matched nicely, along with torque convertor.
Such changes can often result in huge gains.
Last time i saw such big changes was with current car.
Stock shortblock 360 with factory a mile in the hole slugs, 750 carb, 4.10 gear, comp cams generic flat tappet 280 ish advertized cam, stock eddie heads( looked over and checked before use.
This combo what was in my current car when i bought it.
Took it to the track on a good air day, 12.20’s at 108-109

Took heads off, whacked 40 thou off, had them mildly ported in quickie single evening type job, installed a known good 950 carb i had, installed Howards flat tappet closer to what i thought would make more power, and returned to the track with just those mods......
Milled and lightly ported heads, cam swap, carb swap
11.20’s at almost 119 after little tuning....in good air
Good heads, matching cam, good carb, bit more compression...
Full second and 10 mph
Same gears, vert, headers, weight, etc, etc