Someone can correct me if I am wrong, but in those years I think the only real differences between the F-150 adn F-250 were the spings, rear diff, slip yoke, and driveshaft. I think there were leafs on the front rather than or in addition to (don't remember fo r sure) coils. I think it depends on if it was a HD 250 as to if the front axle was different (solid as opposed to independent I beam thing). As far as the engine tranny and tcase I think they all are the same and I think even the frame was the same on the LD 250's. It's been a long time since I messed with these trucks so take this for what it is worth. Bottom line (I think) is that about the only thing you gain going to 3/4 stuff is the larger rear axle and springs. Otherwise there isn't that much difference.