They will fit a 96 just fine! Although they are 5.4l specific heads with 5.4l embossed into the casting, im not 100% sure of the differences from the 4.6l. The non-pi heads also had 5.4l specific heads as well and I know the exhaust port around the valveguide has more area. I dont know if its the same scenario but still very capadible. Buy them up and swap away.
Its a good idea to get the covers...normally, but since you have a mustang and the covers would be truck, the oil filler neck may be opposite from yours. IIRC your covers should work and if they dont, its no big deal. Covers are available windsor or remeo. Easy to get your hands on a set.
Get the heads, buy a head gasket kit, the Ford pi intake manifold and some rtv. Should get you going!