Exhaust Manifold hardware

Going with bolts or studs is fine. I prefer buying grade 8
To answer your questions about washers, you want hardened cone washers:
Mancini Racing Exhaust Manifold Conical Washer

It's important to at least use those on the 2 end bolts/studs on the manifolds.
Or you can just use them on all of the bolts.

If using bolts be sure they're not too long to bottom out! The threaded holes aren't very deep in the head.