It's been done on some 99-04 cars, as said most were GT's.
I'd subscribe to the v6 to 4v swap by bluegt96. Similar, but imo a more feasible swap that is actually happening on the forum, not just a theoretical.
V6 to Mach 1 Swap
12 Black GT500 SVT PP l 2.6 Pulley/Idler l Airaid l o/r X l 305 NT555R's l Recaros l
04 Azure Blue Mach 1 / 03 Sonic Blue 3.8 / 14 Sterling Grey F150 FX4 402A 5.0 / 11 GT500 R.I.P.