Does the engine happen to be out of the car? If not, removing the head so this can be done on the bench will be your best bet to do this without damage to the head.
There’s a couple of places here in Michigan that have inventories of NORS exhaust parts including mufflers. I just bought the correct replacement single for the Mobile Director a couple of months back.
I’d leave it be. Chances are you get that thing moving enough to actually operate smoothly the combo of the pitted shaft and bore will cause a loose fit and an annoying leak.
The right hand will be 2899879 and I don't have an answer for you on the LH. My book doesn't actually have one listed for '72. In 1971 it would have been 3418351.
That is the truth, two perfectly machined surfaces that only have exhaust gasses flowing through don't require gaskets. Today I would say most of the time gaskets are used.