I think I've replaced bolts/gaskets on maybe a dozen manifolds so far, and I'd say out of that many, I've only had to drill out approx. 6-8. Often as not, enough is hanging out when the manifold is off, that a small (5-6") Visegrips will twist them out. With the nuts corroded off, the torque on the threads is gone. Minor corrosion is all that usually holds the leftovers in the head.

If you get a new manifold, and NOT from Ford, it will likely be Chinese, so pick your poison.

Dave