I did away with the factory a/c long ago on my 68 camaro. Now I'm wanting to install a vintage air gen II compac and need to cover the existing holes in my firewall. What is the best way to get decent results without welding a panel in place?

there are plastic covers that will cover the hole, but you will not be happy with the results... I landed up having mine welded up and I'm much happier with the results. Save yourself some time and money, have it welded up.

Or make a 16 gauge panel and use structural adhesive to attach.