While your problem might be the wrong ratio of roller rockers you might have to modify your Vortec heads to accept a high lift cam. The Vortecs in stock form are only good to about .450 lift or so without machining the guides down for clearance.

Check with most any engine shop that caters to building race engines as the Vortecs are very popular for those classes required to use OEM engine parts.

Regards,
Gerry Dedonis