Doug, that's what I was thinking too. I'll do some poking around and report back. Unfortunately, that area got banged up on the old cab, so I'm going to have to either try to get it back into shape, or try to find someone who will let me cut it out of their cab. I would think that piece would be the same on both styles of cab?
I also thought I'd mention that I found another sm319/R10 on ebay and snatched it up. Hopefully this one is in better shape, it looks like it in the pictures anyway (which obviously tells me the inside is in good shape, right? :))
More updates soon hopefully