No Limit Engineering sells a rear coil over conversion kit for 63 - 72 trucks. I could not find it on their website or catalog, but I did get confirmation today that they do sell the kit. The website does have an instruction sheet for installation (that's why I checked to see if they sold the kit). The kit sells for $569.00 and is part #RCC-6372.
I have never bought anything from them, so no opinion about anything.
www.nolimit.net