Cut the cat off or hollow it out, then do the plenum. Driving around with a clogged cat runs the risk of cracking your heads....so you really shouldn't put it off anymore.

 

Cut cat out, drive really loud truck for a couple weeks while you order your plenum and new cat.

Fix plenum, then cat and upstream O2 sensor

 

Well the plugs, wires, cap, and rotor along with some seafoam did wonders for the engine. I pulled the front o2 sensor and it ran the same, so id assume that means the cats OK. I still have the original exhaust on it and my muffler is a rusty mess, so ill have my exhaust shop take care of it this week when i have the rest replaced, I'm ordering the plenum kit ASAP also. I figure I'll just get it all done now. I'm sure just the plenum kit alone will liven it up. If it just takes away the spark knocking it'll be worth it! It uses about 1 quart?3000 mi.

Thanks for all the help