I had an 81 F250 D60front, D70 rear, running 42x18" Ground Hawgs.
I hated the body lift I bought it with and the kits were much more expensive.
Try to find yourself a good spring shop. It was called Universal Joint here but they were bought out by someone or changed their name a couple years ago. Cost was about $200 per spring for 6" custom bent spring. Came with new shackles and poly bushings. It's probably more now, this was almost 10 years ago now.