I’m confused… Are you going to use the stock radio, or an aftermarket?
I admit to not knowing much about '59 radio options, and whether stereo was a stock option; but, here’s a thread about a company that retrofitted my stock (mono) radio with stereo, fade front to rear, AM/FM and Bluetooth.
Simply going to splice the speaker wires together so that you can hear the original radios sound when at car shows or whatever to see that the original radio still works, but also when driving get the sound from the aftermarket stereo.