will a 66 barracuda radio fit a 66 valiant

A little hard to say. The buttons are different & that's probably where the biggest problem would be. You could try it & see. If it doesn't fit & both radios are made by the same company you might be able to switch the button portion from one to the other to make things work. Or work out some other combination of the two radios to get things to work. Depends on how brave you are, going into the works of the radios....