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The A/C compressor bracket on my '88 is cast Al and maybe could be welded. Your '86 has a different A/C compressor so I don't know what bracket is used; if it looks the same color and composition as the Alt bracket I would guess on Al too.
You could likely get a used one from Contemporary Corvette, link on my site.
Mine (85) was cracked when I bought the car - 5 years ago. The new one came from the local chevy dealer, and was fairly cheap - if memory serves it was about $40.