[argonaut]

FF is just like a front axle. There is an axle hub, that rides on a hollow spindle. The spindle bears the load of the truck and the axles can slide right in and out. if you ever have an axle failure, or a differential failure, you're axles and wheels won't slide right out like on a SF.
Thats why I like full floating axles, but they are physically heavier.

Looks likey the 99 springs should be 2-1/2" wide. http://www.sdtrucksprings.com/index....oducts_id=7367
if I were you I'd try to get the springs with the axle. You may be able to make the later model springs work with some poly bushings for the wider hangers. Are your 68 springs 3" wide? Do they get skinnier or wider at the ends? The 99 springs are going to be longer, but you could relocate the spring hangers as well...