What are the part numbers on the GC manifolds you are NOT using? The Red Rock guide I have says part number 53013606AB for left and right. That has to be a typo. That part number is 2005-2008 GC from my little bit of searching. For me the upside of using the GC manifolds is I will probably buy a factory GC exhaust pipe with the cats and I'm sure it will need mods but it will give me a good start. Probably going to run the heat shields on the manifolds too to keep as much heat as I can in the exhaust pipe. No hurry, I know your trying to take it easy.