Caddillac Seville's used 12bolts with 5x5 bolt spacing and discs.
TSM makes a kit for 4x4's and there is someone that is having a Group Buy on rear disc setups for the 6 lug on
http://www.fullsizechevy.com/forums
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Chris Lucas
1973 Chevy C-10
1978 GMC Jimmy (2WD) - SOLD
1987 R10 twin turbo LS
1991 R3500 SRW CrewCab
1985 K5 diesel swapped project
1989 K5 2WD conversion w/ Vette susp Project
Captkaos Customs
73-87chevytrucks.com