4.3 vortec V6 running terrible

[leegreen]

re-read post 12 above
Your exhaust could be plugged.
You have spark. Pull a plug and see if it smells like gas to confirm you are getting fuel to the cylinders. Might even be able to smell it in the oil if you cranked it enough

you said the cat was rattling but you shook the chunks out and it is now gutted. Can you see right through the pipe now? Cat can break up but still be blocking the pipe. Getting a crowbar in there and smashing all the ceramic is the only sure way. Unless you can see right inside you probably need to cut it open.

Drop the y pipe and see if it starts.
I had a 4.2 inline 6 that put up a cat efficiency code one day and stopped dead on the side of the road the next. I pulled the upstream O2 and it started and I drove it home breathing through a 3/4 inch hole. It was down on power a bit though.