I've got a Atomic 4 manifold with a stripped-out drain plug. I've found it difficult to tap the cast iron, so I went looking for some information on how best to create new threads in a hard iron casting. Along the way, I found some interesting web pages:
[http://www.wsavts.org/machine/machine3.htm How to use a tap and die]
[http://www.hotrod.com/howto/97018/ Thread repair how-to] using a thread insert
[http://www.motorcyclistonline.com/howto/122_0312_howto/ Thread repair how-to] by rethreading in place, or using a thread insert
[http://www.emhart.com/products/helicoil/install.htm Installing Heli Coil® thread inserts] and [http://www.airheads.org/old/index.php?Technical%20Tips+Heli%20Coil%20Primer Primer on Using Heli Coil® Inserts]
[http://www.artmetal.com/brambush/forum/bramyak2/messages/27.html thread discussion on drilling and tapping]
I also found this [http://www.ytmag.com/articles/artint89.htm Tinkers Guide to Manifold Repairs] which describes the types of cast iron and the ways to repair them, including welding, brazing and even epoxy repairs.