Have done several this way-hooked up to stock antenna. Have also done this using a factory radio block off plate and hidden antennas (two) Still have the template somewhere.
Don’t they have a kit to put one behind the original speaker grille? Edit# Found it. https://www.hotrodsbydean.com/product/hrbd-40s-stash-box-standard/
I used to think that I HAD to have tunes in my cars, but now I'd bet I could count the number of times I've turned on the radio in the last five years, on two hands. I just don't need it any more, and in fact, it's kind of an annoyance. I'd rather hear the engine, and just enjoy the drive....
I'll never put a radio in my 41 Cadillac. I prefer the look of the original block off plate. If I want to listen to music, I'll just turn on the music on my smartphone and play it through a Bluetooth speaker. Just went out and snapped this picture in the dark. Maybe later on today I will get ambitious and celebrate my birthday by breaking out the Chrome polish and cleaning up the interior Chrome. It's only been 35 years since I last cleaned it!