I wouldn't bother with the factory cast Iron manifolds. I reckon they are probably more trouble than they are worth.
I picked up a pair of factory Corvette exhaust manifolds on ebay for about $80 shipped to my door. These are pressed steel and a lot easier to work with. I cut off the outlet pipe and collector part and fabbed up my own outlet and welded it on. These are the basis for my quiet under body system. You can buy new O2 bungs and weld them in and it was a pretty easy excercise to make these manifolds fit.
There's a couple of different designs of pressed steel exhaust manifolds for the LS1 out there but I think the Corvette ones are the easiest to work with. I've got a pair of Camaro ones as well but they are quite different left to right.
Cheers