340 HP exhaust manifold bolts, where can I get em?

No gaskets needed IF both surfaces still show original surfacing or close to it and are straight. That's how it was done and that's what I've done. Do use the special conical washers on the outboards. Do check the threads to be sure the bolt does not run out of room or thread. I've just continued to use regular hex nuts on the studs even after I bought the special long nuts. So that's worked for me. YMMV.

The maninfold to head pipe does need gasket. Both Accurate and TTI sell the correct size (hard to get at parts stores in my experience). However their gaskets are not the same. I can not say whether one is better than the other, just that they are different.