I noticed that the originals on my '68 have two holes for mounting on the body verses the five in the popular repros.

Mine have probably been repaired/modified multiple times during numerous exhaust replacements I've had done at various shops over their 53 years so they likely have miscellaneous hardware that is different than when they were new. If I were to replace them, I would probably order the ones I see at Gardner that claim to be accurate repros:
https://www.gardnerexhaust.com/exhaust-systems/firebird/1967-1968-firebird-exhaust-system/

I don't know if Gardner will sell them separately or not. If I recall, when I was looking for a crossover muffler, I recall one big vendor who only would sell full systems. I don't recall which one told me that.