Hey Bill,
I can only report on what I found with my head and talking to others, Doc or someone else might have more info. But here's what I meant:
Yes, stock there is not much clearance between the retainer and the top of the seal (maybe enough to run .450 lift? I don't know.) Running positive seals helps. I had my machinist cut my guides down to have enough clearance to get .500 lift and he told me that I could not go much further without making the valve guide so short that it would not support the valve properly. That's why people want to run longer valves.
Sorry that was not very clear - I was talking about if you use longer valves - if the angle between the rocker and the valve gets crazy the edge of the retainer hits the bottom of the rocker. Some guys on SS.org ran into that problem. The beehive springs have retainers that are a smaller diameter, which gives more clearance.