There are some aftermarket deals that have all the modern geegaws(blue tooth, hook up to your phone, connect to a hidden cd player or a digital drive etc.) with them but fit in the original opening.
A car show pal of mine has a '66 Ford F100 and they used one that fits right into the original opening, looks like an older radio and all that, but hooks up to their phone, sounds just fine and they hooked it up to newer speakers that fit in the original speaker locations for their rig.
Similar options are available for our trucks.
I did see a article where a owner took an original radio and wired/soldered in a input jack for his phone, basically turned the radio into the amplifier with the volume knob as the off/on and control for it all.
Do a search for whatever year your truck is: 1964 chevrolet c10 radio
You will find some options come up.
Also, poke around the vendors sites, to see what they sell:
The Chevrolet & GMC Trucks Parts Vendors ListThere are options out there, go explore.
-Woogeroo