Cupped washers

From the factory, AFAIK, for 318 cars; there were only two per side, one on each end, under the fine-thread studs.
but 340 cars, there mightabin more. If they fit, I would install as many as possible.
They were NOT there to solve a sealing issue. Rather to allow the manifold to walk back and forth on the thin shim gasket, and to help prevent the snapping of the fine-thread studs.
I put one on every bolt/stud; six per side.

--------------------------------------------------------
If you had thick gaskets and header bolts, you wouldn't need them. But you may still need a mechanical way of keeping the bolts from backing off. Which is why I have super-thick gaskets, ARP fasteners, and those cup-washers under every bolt .. and my TTI/fat-flanged headers never come loose.

IDK anything about sleeve-bolts.