I went to the local hardware store and bought stainless bolts and nuts when I replaced the cat on my 1982 convertible. They were standard (ie not metric thread) and fit fine. I think they were 5/16" bolts. I may have had to hacksaw one or more to length. Sorry that I can remember details, but I've used standard stainless fasteners to do cat work on the 82 convert, my 95 Golf (you have to disconnect the cat to replace the oxygen sensor) and an 87 RX7 I used to own. They're all running together in my memory.
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