Man if at all possible you should try and go old school 6x9's in that car. Shame most of the speakers made back then even if you could find them new had foam surrounds that would have deteriorated by now like some original Jensen Triaxials.
I have a nice old pair of Cerwin Vega CS-18A HED's with the cast aluminum frames (Made in the USA, not the foreign crap CV is currently peddling), the horn tweeter that is integrated into the grill and a tweeter protector with a reset button right in the grill. These babies were like $249/pr back in '79 - '80 that I could sell you reasonably cheap. They are top mount only though since the solid metal grill is integrated onto the speaker. The surrounds are pleated material with a coating on them so they won't ever wear out. They have huge 6-8lb magnets as well.
If your rear window is at angle that you could fit them in they would certainly do that old muscle car justice.