I have a '93 K1500 5.7 liter 4x4 Silverado. The original exhaust was 3" all the way to the muffler and then two tail-pipes out of the muffler.
I just thought that I'd give a quick description of my experience after replacing my original stock single 3" exhaust with a high-flow catalytic convertor and a Flowmaster 40 muffler, in case anyone else was thinking about doing this.
I knew that my 350 was getting old, but it was getting REALLY weak feeling. It felt especially weak feeling after I went to 285's from 265's on the stock rims.
Some guys I work with had replaced the exhaust on their trucks and reported more power and better has mileage, so I figured I'd give it a shot. It was about $320 to have everything professionally replaced but the Y-pipe, and they welded pegs to use my stock hangers.
I now have LOADS of power and I went from 13 mpg to15 mpg immediately! After adding a K&N drop-in filter, AC Rapid-Fire plugs, and Borg & Warner oxygen sensor, plug wires, cap, and rotor, I'm up to 17 mpg on my old TBI engine which now has over 158,000 miles. It can actually peel out with those 285's now!
If you want to bring your old truck back to life, I'd definitly recommend the exhaust upgrade. It's not nearly as loud as I was expecting, either. At highway speeds, you can barely hear it, so it's not as if you were running glass-packs. I was worried about it annoying my neighbors, but that's not even an issue.
-Derek