ck engine: cat convertor options

98 Safari, 150 k miles, averages about 18 mpg. Power ok, but seems to have dropped off slightly over the last few months with occasional slight spark knock.Check engine light came on, so I had the code read at Advance Auto which said convertor system not working adequately. Checked exhaust at end of tailpipe, and almost no exhaust pressure @ idle, somewhat better when engine is revved. So, if the convertor is plugged and I replace temporarily it with a long-term "test pipe", will this throw the computer off without the backpressure or maybe the higher emissions output? If I do actually replace the cat, can I use a universal one? Advance Auto has the correct fit for $250 or a universal one for $85. I also found universal ones on EBAY for $35-40, with several sizes available. Any reason not to go this way if it's the right size. Most of the read "high output". I've got buddy that can weld it on, if needed. Best course of action is....................? :2cents:

BTW, life-long resident of west Michigan where we don't really have emission tests yet..

chop it off and weld in a new piece of pipe
it wont hurt the computer readings at all