98 Silverado 5.0 hitting P0420 DTC on consistant basis

[inthechateau]

So a few months after dropping new 5.0 into my 98 Silverado it started chugging out a P0420 DTC code. history is the old engine when I bought truck had blown head gaskets, found several cracks in the heads actually so I put a crate engine in it. After starting the new engine the exhaust took like 30 minutes to clear up all the coolant and oil in the exhaust. I am now thinking the CAT is all crusted up with the burnt oil and coolant that was run through it before I got it and needs to be replaced. I don't have the cash right now to put a $350.00 Y pipe with CAT. I do have a TIG welder and am pretty good at it. It looks like a 3" in and 3" out. Can anyone confirm that? Been too wet here lately to do any crawling around , might get calipers under there tomorrow. Any suggestions on a good weld in CAT and source(store)?

 

[inthechateau]

Need ideas on replacing the CAT. Any hints?

 

[inthechateau]

Got tired of the CEL on, no responses or hints here so I took the jump, pulled the Y pipe, and after several fitment installed and tac welds finally go *** right. Welded in a new Flowmaster universal CAT. OEM Cat was all busted up, half full of oil and coolant deposits. Cleaned the O2 sensors and back in the assembly went. I used my 80 amp wire feed welder to put it all together with and seems to be doing well so far. 250 miles and no CEL light and performance has greatly increased along with my MPG.