ORU makes a RSK for the 80-91 trucks
http://www.offroadunlimited.com/orustore/pc/viewPrd.asp?idcategory=850&idproduct=8304 They are more expensive than the SKY one, but they give you a bracket(you weld in) to box the frame to make it a stronger set up.Then you bolt in the RSK.
As for using superduty springs that is a very common occurance. You can use "V" code springs for a soft ride and only gain the lift from the RSK or you can use "X" code springs and get an extra 1.5 inches..Little stiffer ride The SD springs are approx 4" longer than OBS springs so you need to move the RSK hanger forward 4", the rest is a direct bolt in.
When i put my D60 under my 86 i am going to use
http://www.sky-manufacturing.com/new/detaproduct.php?id=162 With superduty "V" code front springs(2.5" of lift) and ICON 5" rear lift springs for a superduty with no block(match the front lift)
O the RSK are exactly the same for 80-91 and 92-97 , just that the ORU one for the 80-91 gives you a pice of metal to box the frame
Here is my one 97 F350 w ORU RSK and stock F350 springs all the way around and a 5" block in place of the factory 4" rear block
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here is my 96 F350 w ORU RSK and 4" superlft front springs(2 leaf removed) and skjacker 6" rear springs w a fitted add-a-leaf (TOTAL ABOUT 8"Lift)