Eric, If you're going to pull the motor anyway for whatever reason, that would certainly facilitate the job of drilling out the broken exhaust manifold bolt. If you had planned to leave it in the truck for a while while you work on it,Here is a trick that I have used a few times with very good results. Assuming that you have the manifold out of the way, there is enough room to get a right angle drill with a shortened bit in there between the block and the inner fender panel. If there is any bolt at all above the block surface, it will help to gently grind it as flat as you can. Then center punch the broken bolt as accurately as possible. Slowly drill a small pilot hole in the broken bolt to a depth equal to the depth of the exhaust ports other hole-you dont want or need to go too deep. Then gradually make the hole bigger until you have reached the drill size for the tap that you will need to clean up the threads. I agree with a previous reader that this is no place for an easy out. On my 59AB flatty there are two different sizes of bolts used to fasten the exhaust maniforls. Your motor is probably an 8BA but You can figure this out. At the risk of sounding a little surgical, I have had success using a mirror to see what is going on during the preccedure. This will take a little patience and dexterity.

regarding headers, I found an original set of Fenton headers at a swap meet that look cool and really get the flow right with the stock steering box. They repop these but they are approaching $250. If you never wany to have to revisit your muffler needs again. I would recommend stainless such as Flowmasters or Dynaflows. I put Dynaflows with aluminized pipe and the sound is awesome. You have a worthwhile project. Keep on truck'in. Lazy