I think the main difference (but im not completely positive) is that the stock heads have an air tube mounted in the back of them that pumps air for emissions. I put my Edelbrock Victor Jr.s on and they didnt have the hole for this tube, so im thinking they are Non-Emissions legal. Most heads will tell you if they are or arent, i know AFR's come in both designs, just get the emissions legal, it cant be real different.
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emissions vs non emissions heads - difference? 
