Exhaust manifold questions

Good morning,
I have two sets of exh. manifolds to choose from, an older set of A body 318's and a later model set from an '88 M body 318. The first question is what are all the extra holes on the later manifold. They were all plugged up from crap and could not have helped anything. The extra holes are circled in the first pic.

In the second pic i have circled the end of the heat riser. If I remove the heat riser can I leave this bushing in and simply braze it shut or will I have to remove the bushing and have someone else weld the cast iron hole up? Would I be better off just to wire the heat riser open?

Third, On the M body there was an O2 sensor on the top of the manifold. So far I've not found a plug that will fit. Anyone know what size thread this is and where I might find a plug? It is not a 3/8" pipe plug.

Last, for now anyhow, anyone use a set of these M body manifolds and have any fitment issues? They are a good bit larger than the older manifolds and should have less restriction.
Thanks,
C