School me on 340 exhaust manifolds

you might find more date code and production info searching the tech forums at moparts.com
In particular look for the thread started by Scott Harms about all the tiniest details and variations on 340 engines. That's in the restoration forum.

As for the extra stuff needed.
Outer fasteners should be studs. The studs are in the water jacket (in other words a little pipe sealant is needed). My recollection is that they are coarse thread into the head and fine thread sticking out. This matters because some? 340s got a special threaded tube with a hex on the end rather than a regular nut.

Underneath the nut, a thick conical washer is used regardless of year. My recollection is that its also used on the outer attaching bolts, but not the inner ones. Verify all this with shop books and actual survivors. The washers helps distribute the clamping force properly.

If you are going this far, you may want reasonably close (or exact correct) head markings on the bolts. Can't help here, but there a people who have collections of bolts they've pulled off cars and are in good shape.

Gaskets. I think 70 had the metal heat shield on one side. No gaskets used (other than that). If the machine marks are still somewhat visible on the exhaust manifold's ports, its probably true enough for a metal to metal seal. That's been my experience. I also check trueness on a flat glass plate or machine surface.

Gaskets part 2: Head pipe gaskets are hard to get in the right size. Both Accurate and TTI have them in stock. Accurate's use a metal fire ring. TTI's are slightly different design. Sealing here is a pain.

Head pipe attaching bolts. Can't help you here for factory correct. Just remember one of them goes in from underneath like the shop books show.