A coule of things, 1, your truck should already be 12 volt. '55 2nd was the first year for 12 volt in Chevrolet. 2, you're not going to find a radio that will work with the Dodge harness. Your est bet would be to forget the oem Dodge harness, use an aftermarket harness for the engine controls and a proper body harness for the cab/lighting that's specific to the '55 or a Pinless universal kit. The radios that are made to fit the '55 dash will come with their own separate harness, ad only need to be hooked up to a power source and the wiring for the speakers.
Custom Autosound has several stereo systems that directly fit in the oem opening, no cutting required. There is also another company, I don't recall the name, that has a direct fit stereo that matches the oem almost perfectly, but they are rather expensive. I have a custom Autosound in my '55 (cabs are the same) and am very pleased with it. Speakers that fit in the stock location above the mirror are available.
