If it's just a hair too big I would try spreading the mount with a big pry bar and lubing the bushing as I force it in.

 

I just did the upper control arms on my '02 and they were the exact same way. Rear arm slipped in fine, front too tight. I finally got it to go using a "plastic" dead blow hammer. I believe the bushings on the new motorcraft upper control arms I bought are the same as yours. Here is a picture of the drivers side front that needed tuned up via the dead blow to get it in. I did smear a little bearing grease on the walls to help too....

 

I don't have it in front of me or handy at the moment but IIRC the torque specs for the 2WD upper arm nuts is less than 50 lbs. and even less for the 4X4's. That's not a whole lot. Regardless, I've seen plenty of arms go on the truck with an owners TTY spec. Your problem may be that simple. Spock always stressed the simplest solution.

 

Thanks everyone for the input. The UCA was installed successfully with the assistance of a buddy of mine and some directed percussive persuasion.