Cat or Muffler

If you could do just one which would give you the best HP. and mileage a high flow cat or a after market muffler. I guess what I'm asking on a stock exhaust which constricts the most air.

On most vehicles a performance muffler and hi flow cat flow about the same air. From a performance standpoint they will do about the same. Figure 2-5hp for either. Straight through mufflers flow a little better and if a motor has a higher hp rating, you can expect maybe a little better gain. Now compare that to mandrel bent piping replacing an exhaust that had compression bent piping and you will gain anywhere from 15 to 50 hp! The big gains are in the piping, not the muffler or the cats. That being said, I'd do the muffler because it's cheaper and doesn't void your warranty.

My stock exhaust on my 88 looks so restricted! The factory y-pipe is more like a
T-pipe. I plan on keeping factory manifolds but from a 92 or newer. According to one of the other members on this site, they will have 2-1/4 inch outlets. I want to go 2-1/4 all the way to the new y-pipe then into 3 inch the rest of the way back. Mandrel bent, free flow cat (or cat delete). Then a quiet but non-restrictive muffler. My days of LOUD exhausts are over. Less attention from Johnny Law, plus I don't need to sound like I'm going fast. It's a truck, for Pete's sake! I just want the Gray Goat to breathe a little better.