Stock magnum heads cam roll call!
Retainer to guide clearance is the issue before valve to piston clearance on a stock piston, stock head and valve magnum setup. I have that Hughes cam with their retainers, locks, and springs. I had to shorten the guides to get adequate retainer to guide clearance. With the guides down .100", you have clearance for a more aggressive came. Details from my measurements are in another thread.
How fast can you go with a Magnum? Who has the quickest setup?