If you wanted the 4" rancho. i would honestly just do a body lift. it will give you pretty much the same amount of clearence, for alot less.
from what i've seen a few people use, was some thick rubber to fill them in. is what i would use. Just make a template, cut it out. And secure it in. But you could use anything i would imagine
X's 2 the ride stays the same as stock. your not touching anything with the suspension. I am rocking one on my truck and love it. you can also buy the gap guards but you could make them for alot cheaper.
I test drove a '05 F-150 last year i was lookin at buyin with the 4" Rancho lift and it looked real good. As in the lift did, and it drove amazing. Had 35" procomp a/t's and procomp rims. But anyway, i can vouch that if you put it on right it'd be a good quality lift. We spent half a day with it at our shop inspecting it and looking it over and it was pretty nice. Shoulda bought it