I put dual four inchers in my Barracuda and Valiant. One option for the early As is to make a plate that mounts to the regular speaker bracket. I even found room to mount small tweeters in the Valiant with the crossovers sitting on a bracket on the radio. The Barracuda has coaxial two-ways. Both cars sound good and have surprising stereo separation bouncing off the windshield. If you do it you can prevent over driving the small speakers by mounting a filter capacitor. It keeps the small speakers from trying to create too deep of frequency and buzzing or popping due to lack of control.
496 MFD blocks below 80Hz @ 4 Ohms
398 MFD blocks below 100 Hz @ 4 ohms
330 MFD blocks below 120 Hz@ 4 ohms
This is an approximation so capacitor values can be slightly different.
Keep the magnet size and location in mind as they have potential to interfere with the wiper mechanism so check prior to final installation.
You can hide some 6" or lager subs somewhere else in the car if you really need the bass of a larger driver.
I'm pretty sure 74Sport has a one piece aluminum bracket that holds twin 3-1/2 speakers as I sent him a pattern a while back.