I went to install my straight water neck i bought from S&P, but it's way to big to go into the hole where my old stat & housing was. My motor for the swap came out of a 2004 GTO. If they are different, does anyone know who makes one that does fit? I saw that CSR makes one, but i'm wondering if theirs will be the same and i end up having the same problem.

Thanks

 

The 04 GTO uses has the thermostat integrated with the water neck.
The 05 / 06 LS2 GTOs use the later style thermostat that is separate from the water neck.
With the issue you are having, I would think you may have a water neck for the later LS2 style thermostat.
"IF" the water pump castings are indeed different from LS1 and LS2 GTO (and mybe all LS waterpumps) you may want to use the later LS2 style waterpump so as you can use your existing water neck.
--Glenn

 

Here ya go.
LS1 thermostat and housing dont fit into the LS2 water pump without mods.
https://ls1tech.com/forums/conversio...ter-pumps.html