Since the original bolt has a flanged head, three or four flat washers on the replacement bolts would likely be all that's needed.
Verify that there's no bolt shank coming through the exhaust manifold, and that you don't have excess thread engagement. Easily measured.
I would be sure to add anti-seize to the bolt threads, and to clean-out the threaded holes in the cylinder head with a thread-restoring (NOT a thread-cutting) tap.
If the manifolds are flat and smooth, and the head surface is flat and smooth, you most-likely don't need a gasket. But none of my Vortec core engines came to me with exhaust manifolds, so I don't know what GM did or didn't do in regards to gaskets. The Vortecs seem to be prone to cracking the iron exhaust manifolds, and the Dorman replacements will not be useable out-of-the-box. Every Dorman exhaust manifold I've ever seen has been warped and with rough machining. I have heard--but not verified--that the Dorman Chinese manifolds are actually better than the Dorman Vietnamese manifolds, which must be truly horrible.
Verify that there's no bolt shank coming through the exhaust manifold, and that you don't have excess thread engagement. Easily measured.
I would be sure to add anti-seize to the bolt threads, and to clean-out the threaded holes in the cylinder head with a thread-restoring (NOT a thread-cutting) tap.
If the manifolds are flat and smooth, and the head surface is flat and smooth, you most-likely don't need a gasket. But none of my Vortec core engines came to me with exhaust manifolds, so I don't know what GM did or didn't do in regards to gaskets. The Vortecs seem to be prone to cracking the iron exhaust manifolds, and the Dorman replacements will not be useable out-of-the-box. Every Dorman exhaust manifold I've ever seen has been warped and with rough machining. I have heard--but not verified--that the Dorman Chinese manifolds are actually better than the Dorman Vietnamese manifolds, which must be truly horrible.