[updated:LAST EDITED ON 01-May-02 AT 02:34 PM (EST)]My 4.2 is 6 quarts with a filter change, which puts it halfway between the add and full marks on the stick - and I'm pretty sure my manual says that the 4.6 and 5.4 are also 6 quarts, at least in the F150 for the '01 model year. However, the pan used on the F250 might be slightly larger - using 6 1/2 quarts doesn't seem unreasonable. Personally, I would just start with putting in 6 quarts and see where that is on the stick (once the engine has been started and the oil circulated, and then allowed to run back into the pan for a few minutes). I think the sticks are pretty close in the newer engines, as opposed to some of the old 302s where they were only accurate within a quart or two.
BTW, most of the Fords I've owned did have "add" and "full" marked on the stick - as do both the '89 and the '01 I own now.