I good ported box would definitely increase the SPL inside and outside of the car. But the ported box would also be bigger than the sealed box, so are you willing to lose more trunk space???

But before you get to all of this, maybe you should look into different subs. XLC's are old and weren't exactly Rockford's top subs back then....

I'd suggest a single Kicker 15L7 in a 4.5-6 ft^3 ported box.... My bass carried for atleast a 1/4 back in the day when I was running a 15L7 in a 3 ft^3 box in my 94 LS sedan. That car rattled like crazy too, but the bass was still very powerful even with the trunk fully opened.