What exactly are you lookin for? 390/319, 427, 428, 460? Or are you looking for a 477/534? Those will be tough to find in even rebuildable shapem and will be expensive. I'd go for a 460 mildly built (will cost about as much as a 454 to build).
I don't think any of the Super Duty V-8's would make for a good swap into a pickup truck. These engines are very heavy, turn slow, don't breath well, and only bolt up to big truck clutches and transmissions. On the other hand, it would make for an interesting truck, and you would be a big hit at Ford truck shows! Ford never installed these beasts in anything lighter than a 750 series truck, and that should tell you something. G.M.C. did install their big truck V-6's in pickups, but the V-6 came in smaller displacement versions like the 305. And, the G.M.C. V-6 was smaller and lighter (but not much lighter!) than the Ford Super Duty. However, those pickups with the V-6 were not known for fuel economy nor did they rev. very fast, and G.M.C. customers preffered Chevy small and big block V-8's when they became available in the later 60's.
I figured since he was posting in the SD forum, that he might have a vehicle capable of handling an SD, and was just looking for a replacement. I totally agree that in a pickup smaller than a F750, the SD is NOT the way to go. A 390, built how you want it, would be great.