1998 Chevy Tahoe Oxygen sensors

The compresson test was the first thing I did after I cleaned the cats out Cause thats what I was thinking also. All cylinders had good compression going by my haynes manuel. A mechanic friend of mine also checked the compression just to make sure and He said it was perfect. It runs like its out of time but its not. All plugs have a black carbon look to them and if you let it idle for a couple of minutes it will foul out the plugs. Also white smoke is coming from the exhaust. I've changed the entire distributor including CMP sensor, the fuel injector pack, Crank sensor, MAP sensor, ECT sensor, Coil mod driver, the coil, all plugs and wires, The computer, fuel filter, checked fuel pressure(60psi), Pulled timing cover to check out, checked vacuum lines, checked resistance on all sensor wires, checked and cleaned all grounds.It idles rough, the tach jumpsall around the 1000 rpm area and it backfires loudly, one after another.

Its got a flow master on it so I'm thinking it would of blew any water out of it. But what I can check is when the cats shattered if there is any of that junk stuck in the muffler. I checked for vacuum leaks but I didn't check vacuum pressure. Is there a way to check for back pressure. Do I need to get a speacial tool to check to make sure I'm getting the right intake vacuum and is there a set vacuum # I'm looking for.

Ok I did some visual checks on the exhaust and here is what I've got. Can't see anything inside muffler and when I shake it I don't hear anything moving freely inside. What I do have is on the back cat the gasket where the pipe bolts to the cat is blown out and the other exhaust pipe is still cut behind the front cat where I cleaned it out at, but its a hack saw cut so there is no gap, but still a major exhaust leak. I pulled all 4 o2 sensors and one of the back ones has a hole burned clean threw the outer casing on the out side of the pipe and the other back one the inside probe part looks like someone took a hammer and tryed to beat it flat and to one side. Both front ones look alright but they all have a alot of rattleing inside them when you shake them. Is that normal. Could the exhaust leaks and bad o2 sensors cause any problems like I'm going threw or is this just small stuff that can get fixed later. Hey and thank you very much for your time.