Swapping to 4V heads is a whole new can of worms. This will definitely take your project over the edge of the abyss, especially since you already have donor cars and parts bought. The 5.4L also uses a different flywheel bolt pattern than the 4.6L engine so you will need to find the right flywheel if you are gonna use a different
tranny that came with the 5.4L motor you have. Also, exhaust is different. Clearance is very tight. If you plan on putting a hood on your car then you cannot use a truck intake manifold. The 4.6L and 5.4L do not share many parts at all. What I should say is that you can bolt many parts between them, but there are still differences that hurt making them work together. THe list goes on. Google it up and you'll find a few talking about it and most never get them to run right.