Im no expert but id say the f350 is worth at least 6-7K, I assume its a diesel.
what year is the F450?
With the F350 you may try putting an add on craigslist and be willing to trade for a newer F150, No doubt you will get people lowballing you and trying to trade their old and busted 1/2 ton for it, but you may find somebody that is looking to get into a diesel (assuming it is a diesel) truck and out of a gasser that will be willing to deal.
Freshman at Pittsburg State University
Encore Walkbehinds, Great Dane Surfer
Stihl Trimmers, Blower, Chainsaws
98 Toyota 4-runner
85 Ford F350 Diesel, 10'x 7.5' Dump Bed
94 Bobcat 7753