On mine, no modifications were performed at all except installation of Hughes cam SER1418ALM-10 and their retainers and locks. I used light checking springs and a set of dummy lifters I plugged to make into solids, and found right at .550" test lift the intake retainers contacted the factory style valve stem seals. The exhaust retainer contacted the seal at .540" test lift. I had my guides cut down for clearance.
Others have said they successfully used a bigger Hughes cam with same retainers and had no failures, but did not check clearances.
When I measured mine, the retainer to guide clearance was an issue way before valve to piston clearance, with stock pistons.
