Matt:
Seans right, the LS6 head is not all that uncommon and the bare casting should be in the $200-250 range. The "LS6" head casting is used on all the new 2007 5.3l V8 truck engines, the FWD impala engines, the LS2 as well as the 2005+ 5.3L engines in the Envoy Denali's, Rainiers, and V8 Trailblazers.
The $$ value in the real LS6 head assemblies are the sodium valves. If you have those in the damaged head, then just get new castings..
How are those spare GTO wheels holding up?