Bish, I've heard this is a common occurance when Chevy air is blown through Ford pipes.
More relavant...when the bottoms of my sidepipes rusted out I had the old mufflers cut out and new ones welded in at Airport Welding in Snohomish. Pretty straightforward and way cheaper than buying new sidepipes. I then took the sidepipes down to Performance Coatings in Auburn for ceramic coating. They turned out great. This is the only pic I could find the shows the sidepipes.
Rich
Here's the link to the coating place:
http://www.performancecoatings.com/