Factory exhaust dimensions on ’67 Newport base w/383

I'm looking to replace the muffler on my car with something better than the old stock muffler. I put on a 4 barrel last fall but I just don't have the $800 to have duals put on right now so I thought I could swap out the single muffler to get it flowing a little bit better.

Whats the pipe size? Any recommendations on brands?
Author: OneEyed