Snowman,
You can get those clamps at any junkyard where you can find an inline GM. The bolts are a standard thread and I recommend replacing them with studs.
If you buy an after market manifold(s) you'll probably need to modify the clamps anyway.
If you Dremel doesn't work (which it should) then try a 'cheater' socket made for removing bolts with rusted threads. Craftsman, Snap On, Mac Tool, Matco all make them. They are invaluable at the junkyard with a breaker bar.
