@memike and
@66340SEDAN may be able to help. They both have 66 Valiant sedans with bigger rears. There are many 66 Valiant owners here that may chime in as well. Although not a Valiant, I put 15x7 4" backspace wheels on the rear of my 66 Dart. I kept the original 14" wheels up front. Here's a photo and a link to my thread.
New 15" rear tires
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