Vorshlag LS1 swap headers, like many high end header kits, do not come with new bolts or gaskets. Some folks like to try to re-use the OEM exhaust manifold studs, but those things are a nightmare to use with full length headers.
If you want to use our LS1 headers, we highly recommend buying proper header bolts: Stainless steel, with 12-point flanged heads, made by ARP in the USA. This particular part number is made exclusively for use in LS1 cylinder exhaust flange bolt holes.
These have the right size, thread pitch, length and strength for this use, and we have used them on countless in-house LS1 headers here at Vorshlag. Add a dab of anti-seize to each of these bolts before threading them into aluminum heads - this prevents dissimilar metals (steel bolt and aluminum cylinder head) from fusing together over time.