56er, the Olds quide plates are for 5/16 pushrods as Tom said; the mounting holes for the plates are also undersized and require opening up to 7/16 for the screw in studs....the Comps that I used are hardened and "opening them up" turned out to be a chore (hard on tooling). Isky makes an adjustable quide plate for the SBC that will widen enough to fit the 6 cylinder valve centers; I'm going to try that route on the next head to reduce the hassle......fats
