I would Say a 750 CFM With a preformer Intake from edlebrock... since you say your gonna get headers..A nice cam for this setup would be part number 32-224-4 From comp cams This cam is great for mild street cars . Stock converter, 3.30-4.0 gear, headers. and has a nice Mild-Rough Idle..And comes with kits..It is a nice cam for these low CR engines.. I know what you mean When your talking 4v heads and Gas prices OUCH!!!
But also if your talkin 500Hp I would upgrade Heads and CR components like pistons and bigger Cams..