If you have a big enough engine with the right mufflers you have all the music you need!
Generally speaking with stereo swap outs when the term direct fit is used that means it fits the opening without modification. As for the wiring you may have to do some modifications or some venders can make you a direct plug in wiring adapter. Granted on these old cars the radio wiring is not very complicated as you generally have main power and speakers. Looking at the options with the Hermosa though you will need to find a second source of power for an always hot so the radio can maintain memory and such. Then it also has some amplifier and such outputs also and since your car didn't have any of those from the factory they will have to be taped up if not used or custom wires ran for them.