Cracked Exhaust Manifold Questions

[A.Bursell]

Hello,

Background:
Finally getting my 67 C10 w/ L6 250 ready to drive. There had been a nasty off-idle stumble. Changed the points and set it up and it started and ran and drove great. Got rid of all the problems. I was loving it for a day. I was so excited that I went and changed everything else you would do in a good complete tune-up (plugs, wires, cap, rotor, air filter, fuel filter, pcv, breather, even a new coil). Much to my disappointment I started the truck and instantly it had a bad miss really noticeable right off idle. Worse than anything before. I started changing everything back figuring something was bad and it was still there after putting all the original parts on. I figured I'd start looking for a vacuum leak and when I started looking around I saw a big crack in my exhaust manifold. Right near the center.

Questions:
Could this crack in the exhaust manifold be causing this miss? Or is it likely that I have multiple problems? Do I need to find a good used one or are these produced anyplace? Or, is there a better alternative to replace this with-- that would bolt on and not require extensive modifications.

Thanks,
Adam