Not that this issue may apply to your situation but I thought I'd throw it in here just as a "double check". My LT1 was running similar with the backfiring and fuel mixture running all over the place. The problem? One burned plug wire (#8) hidden behind a header tube. That frustrated the hell out of me. Doing a meter check on the wires showed that they were all ok. BUT...the damm thing was trying to ignite a header instead of a plug...LOL

 

The plug wires are Taylors and are held in by a bolt-down wire seperator that goes under the valve cover bolts. I'll run an ohm check on them, but they're not close to the headers and I don't know how they could be bad. Thanks for the suggestion.