Here is a quote from the Sallee Chevrolet site:

Stock Vortec heads come with pressed-in 3/8-inch rocker studs and do not use guideplates. The rocker arms are guided (also called “rail” rockers) and employ a pair of guides or rails that center the rocker arm over the valve. This eliminates the need for pushrod guideplates. GM Performance Parts, Crane, and Comp offer self-aligning stamped-steel and roller-rocker arms that will bolt right on. Or, you can modify the heads to add screw-in studs and guideplates and then use conventional rocker arms, but be forewarned that this can add over $200 to the cost of the heads. Do not use guided or self-aligning rocker arms with heads that use guideplates, since this will cause pushrod bind and excessive wear. Also be aware that 1.6 rockers move the pushrod very close to the pushrod hole. Elongating the pushrod hole should be considered essential when using 1.6 rockers.............All of this conspires to limit total valve lift to a safe figure between 0.420 and 0.430 inch. The limitation is the clearance between the bottom of the retainer and the top of the valve-guide seal.

Could it be that you have too much lift for the stock springs??? Sounds like it might be so. They sell heads modified for higher lift cams. OR, are you using 1.6 rockers? That would surely bump the lift up to the danger zone.