I'm kind of a symmetry nut. If one speaker has the enclosure, in my car, the other would as well. Not to mention, a 6x9 speaker, running off of the stock head unit, with no other amplification, would sound really tin canny without an enclosure. My car is being restored to be a weekend road trip ride. We are keeping a LOT of it as factory as possible while also looking to improve on some things. When it is viewed by others, it will "look" to be completely stock. But underneath the "skin" of everything, there are many many tweaks. Including an awesome sound system and a 400+ HP 403. Mind you... mine is not a Trans Am. Its a Formula, that was ordered new by my step Dad. He drove the wheels off the thing in it's first 12 years of life. After that, it was parked, outside, until January of 2019, when we began the process of breathing life back into it.
Sounds like yours is a much less involved project.