273 Exhaust

what dual exhaust? if you are referring to stock dual exhaust, that didn't come until 67 383, maybe; or 68 340/383, for sure.
Even the hypo 273 in the early a-body was just single exhaust, but they did have an optional relatively large diameter single pipe with a throaty sounding attenuator, I think.
If you want a custom dual exhaust that's reasonably priced, that may be up to you to haggle a deal with a local exhaust shop, like previous posting suggests.