Max lift on stock guides and seals?

Most heads I've checked will handle right close to .500 lift and like Tony says due to the losses associated with less than advertised rocker ratios and pushrod angle loss a .510 or so cam will work fine. I did see one set of heads, 1973 587's I believe, that the retainer hit at .450 lift. Not sure why it was so low on that particular set of heads but it goes to show you it should always be checked.