It's more of the drop than the 20's as far as the wheel wells being filled up in my opinion.
Don't get me wrong, when people go to the 20" Lightning replica wheel they do gain close to a 1/2" in the fender. That is when they go from a 295/45/18 to a 295/40/20 it is almost an inch taller so half under and half over or about a 1/2" in the fender.
Here are a couple of pics to give you some idea, and this is with the BellTech 3/4 kit set at 3/3 or 2/3 depending on how you look at it when going from the front SVT spring which is about a inch lower than a regular F-150.

Bill

BellTech 3/4 with stock 18's


BellTech 3/4 with 20" reps


Here is some 22's with about the same size tire, the gap is about the same as the 18's and the 20's. So unless you go with a taller tire you are still going to have about the same from your fender to the top of your tire.

 

Thanks for the reply joe. If I want to do a 2/3 drop, can I replace my rear shackles with 3"'s and then some how drop the front 2 inches? Thanks, Shane

 

hdmlnium your 2 pics don't look that different with the drop on both shots, but the 20's on the 2nd one, it doesnt "fill the wheel well" as much as I thought. So it is a combination of the drop and the rims if I want it to look like that, and Steve's original pic i posted had 20's and the 3/4 drop I think. Even with 18's on it, I like how it looks a lot after the drop. thanks for the info.
Shane

 

Shane,
I have Belltech 2" drop springs on the front (I wouldn't go any lower on a daily driven truck...You don't realize how big our front facias are 'til you drag a curb while parking ) and Re-arched(1.5"drop) rear springs out back coupled with 2" drop shackles to give me roughly a 2-4 drop. I reccomend a good set of shocks(I run QA1's, but there are some cheaper alternatives out there.) I run 18" wheels on my L because of the better tire selection as compared to 20's. Also, they don't make Drag Radials for 20's . To see a pic of my L click on the www. bar at the bottom of this post(check sig).

Later,
Rob