Jones Exhaust also makes pretty much the same muffler as the Classic Chambered. They are available through Summit Racing. I put a set on my car and they seem to be fine.
If you weld in new mufflers, the remaining ceramic coating has to be stripped and and the sidepipes re-coated.
You may have to do a little carb re-tuning if you install new less restrictive mufflers. If you're going to the trouble and expense of cutting, welding and re-coating, you might as well put in some good low restriction mufflers.
Ted
http://www.jonesexhaust.com/products.cfm?ProductTOC=10