I had the same problem as you. My collector bolts to the muffler? with 8 bolts. I tried SS bolts and the lock nuts with a ding in them. I also put on a custom gasket that I carved out of flat stock. When I finally got it apart this past winter, the gaskets were just pieces, and all the bolts were seized so tight I had to cut some of them off.
I got replacements from McMaster-Carr, go to:
www.mcmaster.com
Enter the page number, 3138 into the search box. The high-temp bolts and nuts are on that page. Some are pretty spendy but they work.
I then had Remflex make me a custom gasket out of their cool material. Cost me $75 for the custom gasket but it should work. I also bought their header gaskets since I had to replace mine anyway. Go to
www.remflex.com
I also ground the welds flat on the collectors at both ends because the gloppy welds were preventing a good seal with the gaskets.
We'll see how it works when I get my new motor installed shortly.
Paul