Hello,
Was just reading your post this morning. I have a '73 340 Cuda, I installed the 50 series Flowmasters 2 1/2" in & out with a X-pipe and headers . Sounds nice, not to loud and not to quiet. I also had those 3 disc Car Chemistry collector inserts laying around . So I put them in and it got really quiet, kind of sounds better if your going to do highway driving, I take them out when I'm going to the track. No big deal to change.
Back to the mufflers, they are pretty close to fit but worked out ok, just make sure you weld everything up so you have no rattles....
Good Luck........