Hey this is a really good question. I'm not sure I can help, but here goes..
If it is a GM replacement or NOS there should be a number stamped on the muffler, I think it is usually on either end of the thing. That would be the GM part #. My Flowmasters have "Flowmaster" raised lettering. There are so many different types of mufflers out there. There are the round ones.. "Cherry Bomb" , there are Thrush and the list goes on and on.
If you have a local Muffler shop, a quick trip to their shop and one of the "Old Timers" look at it might be the fastest, bestest way to figure it out.
If there is a # on either end of the Muffler a trip to the local Chevrolet Dealers Parts man might shed some light.
Good Luck and PLEASE let us know HOW you find out what you're running for Mufflers. I for one am curious..
The Mrs. is going on a trail ride tonight, so I should have time to crawl around and get down and dirty. Xena likes that, you know. They look stock to me, but I'm sure they are not. Probably a Midas special. Sounds good now, wonder what Flowmaster 40's would sound like?
Check out Ravin mufflers! They are sold at midas, but they are performance mufflers. They have a 33 series which is supposedly better cfm than the flowmaster 40's. They do sound better than the 40's though, and that is a hard muffler to beat.
I think midas have little dimond shapes stamped into the casing and I know the mufflers i have have the word 'Turbo' stamped into the outer shell and they came from a replacement kit from Mid-Amer? Hope I helped if at all. :s