I also used a 66-67 B body rear in my 80 F body with the same year B body main leaf and the balance of leaves coming from SS packs. These years are 59.5" wide. 68-70 are 60.125". This may create possible fender lip interference depending on your choice of wheel and tire. I also dumped all the iso-clamp system for conventional spring eyes and shock mounts.

I had 15x8 Dragstars with 4.5" backspace and a 295/65x15 tire. Had to severely roll and hammer the fender lip up and it would still kiss the edge of the tire on dips. You can see the thin shiny streak on the shoulder in the pic below.