I wish I could give you a bunch of specific details, but I can't.
I seem to remember about 4 or 5 years ago, several guys here did a lot of modding to their 4.2. You might be able to find something by searching this forum for "camshaft".
I poke a round and see what I might be able to find.
You might try checking with some of the FTE sponsors. Maybe some of them have something to offer.
I believe the Mustang 3.8 shares the camshaft with the 4.2, so maybe find something related to Mustang hop-up.
I really wish I could be of more help, but most guys here seem to just use their 4.2 F150 as a regular old daily driver kind of truck and don't spend a lot of time and money building the engine up.
edit to add-
check here:
http://www.ford-trucks.com/forums/23...light=camshaft
post #19
http://www.ford-trucks.com/forums/64...light=camshaft
post #10
That's about all I could find