Better flow,more power,tempted to drive a little harder. Efi or carb?
If carb, i would check choke/heat riser and air cleaner.
I know with efi,if headers are installed the flow will carry out unburned fuel as its injected before exhaust valve closes. But 1/2 the mileage? That’s a big change. May have to flatten exhaust pipe so its as restricted as it was before. Kidding.
And some older fords took a while to relearn after dramatic changes were made. A battery disconnect would help on some.
I cleaned an iac on a 5.0 a couple weeks ago, went from stalling cold to a very high idle. A few days of driving and it adapted.
