Vortec heads (at least the current GM aftermarket version) have exhaust ports that are raised relative to the header bolt holes. I have found that most headers and gaskets tend to cover the top .125" to .150" of the ports, which is exactly where exhaust flow is the most intense.
These are the brand new GM Vortec "062" heads I used a few years ago,. You can see how much the ports are raised above the horizontal centerline of the bolt holes. Gaskets in picture are old school Fel Pro metal to show the offset relative to traditional heads. I ended up using Patriot headers along with 1.5" round port gaskets by Mr. Gasket. I still had to take a little meat off the top of the gasket port holes and some of the header ports. Thankfully there was enough flat mating surface on the headers and gaskets to ensure there were no leaks. Wish I would have taken pictures of the heads with the Mr Gasket gaskets.
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I can't recommend the Patriot headers because the front tube on the passenger side hit my hard lines (brake, fuel, vent, and return). I had to remove the fuel pump and motor mount bracket to bend the lines out of the way. Patriot (Pertronix) pretty much ignored my situation and actually suggested I move the engine a little to the other side! I think they were designed for 67-72, but the geniuses at Pertronix decided they were close enough for square bodies. I'm done with Pertronix/Patriot/Doug's forever!
I have since learned that Hooker has a relatively new P/N header that is designed for our trucks with Vortec heads. Check them out. The Hooker guy I spoke with was one of two guys at header companies (the other was a Sanderson distributor) who were aware of the Vortec port issue. The other guys are just cruising along blind and dumb!
Ah, just found the picture below. Fel Pro gasket for 96-up 5.7 liter Vortec engine. I used them with some Dorman cast Iron exhaust manifolds that I had to hog out at the top, even though they were listed for 96-up 5.7L engines.
All that said, I have seen pictures of earlier production Vortec heads that did not have the raised ports, but just about every used pair you find will have hairline cracks somewhere.
Whew!
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