what size lift(susp. and/or body) would i need to run 39.5 tsl's on a 76 f-250 and a 79 f-250 with bushwacker cutouts already on the truck?

 

The Highnoy should be able to run 35s stock with no rubbing, and since it's got bushwacker cutouts, put a 4" lift to have extra room to play with. Or just do a shackle flip in the rear, and get some 4" lift springs up front.

 

On the 76, I agree, 4" with those cutouts would fit them fine. On the 79 you'll want something closer to 6" to fit the 39's comfortably.