Vortec heads

does anyone have experience with vortec heads? I have a few questions regarding clearancing for a larger cam. I have read that machining is needed for more lift but have also heard that with use of different springs you can get away with .525+. Any one know?

Yeah something to do with machining either the spring seat or valve guides down to accomodate a larger spring for higher lift.

iirc it was much past .480 and you needed to machine down the valve spring seats to obtain the clearence.