i have a 87 and i have looked at the 73-79 f150's and compared it to mine and there are some big differences. the 73-79 dosent have caster camber bushing and spindels which makes it stronger than mine, yet also harder to have alighned if your caster camber gets out of wack.
the 87-96 beams are longer, i think, i didnt put a tapemeasure on them to see for sure.
if you did do this you might as well buy a whole parts truck and pull all the parts you will need.
as for the motor swap...i would try and find the closest to 96 as you can because not only did they swap to MAF they also put roller cams in them then and they made more power stock. my 87 has MAP which has hardly any adjustability as far as adjusting that stock power settings and makes it harder to add preformance upgrades.
now that im thinking about it, the fox body mustangs had all that stuff throughout their production so unless you just absolutely want a TRUCK motor i'd see if you could find a mustang motor/wiring harness and all. and as for it being a truck specific motor you could get a roller cam and put in. just depends on what you want and can afford really.