Depending on what speaker you have in the dash, there are several upgrades that are easy to do. No door cutting necessary. I did this in my 66 with a homemade speaker holder. It went in from below after loosening the heater controls to get good access.
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There are a couple of options to mount a pair of 3-1/2" speakers, but before buying one of the metal templates, pull your speaker first. There are two shapes for sure (and maybe more, IDK?) to fit the dash. Here are the two plates I have, laid over each other so you can see the difference. The white one is for 4" speakers and mounts from below, the black one is for 3-1/2" speakers and at least in my car, it went in from above.
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This one went in from above, it's in my 72 Coronet.
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If this looks like more monkey business than you're interested in doing, Crutchfield sells a single 4x10 with a dual voice coil. I have one in my truck, it's surprisingly good.