I have not tried it on a 460, but I ported the heads on a 351W and changed from a single exhaust system to a true dual exhaust. And it didn't change the MPG at all. The thing ran like a bandit and sounded really good, but the significant increase in exhaust made no difference at all on the MPG.
Rusty: '81 F150 Ranger XLT 4x4 w/a 351M, RV cam, Performer carb & intake, C6, & 3.50's
Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads, Weiand intake, Street Demon 750/ZF5/3.50 gears w/Kevlar clutches
Worst fear: I die and my wife sells my trucks for what I've told her I have in them.