HP manifolds with NO gaskets (?)

No gasket needed on the 340 manifolds. Factory 'gasket' was the sheet metal heat shield whch few of us have or use. One side only IIRC.

Look at the manifold mating surface, if you can still see some of the machine marks, then the finish is about as good as it was originally. Next check for flatness. If that's OK, then just bolt it up as shown in the FSM. Get the conical washers if you don't have them.

Use a gasket on the outlet. Accurate and TTI both sell correct size gaskets (often hard to find locally). Accurate's has a metal ring - I personally like that.