If the new O2's were put in right when the headers were installed, then some of the crap that burns off the new headers may have contaminated the O2's. As for the y-pipe hitting the floor, mine does slightly on the LS1 as well. New poly motor mounts will limit the engine twist and might solve your problem. I wish I did them when I did the headers because it's going to be a huge pain getting in there now.
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69 Z28 JL8 4 wheel disc brakes - being restored
09 Silverado Z71
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