I've always run an 11 inch clutch on my trucks;there's plenty of room for the lower bolts.The 11 incher has been available for a long time in pretty near all GM's cars and trucks,and for my money it's worthwhile to go with the biggest clutch I can get.(I know-it's overkill,but if it makes the difference between having enough grip or not,even once,it's paid for itself.) BTW-my parts man tells me an LUK 12" diaphragm clutch will fit anywhere an 11" will,and is less money. The part # he gave me is 04-064.$148.13-Complete kit. Might be worth a look.
Speed
