I have a stock 1965 Coronet with 12" ( 305 mm ) wide Mickey Thompson Sportsman SR on my rear with 10" x 15" x 4.5" backspace wheels. I had to install new springs to keep the rake of the rear of the car up ( not a lot though ) they fit and look great.
Have a picture form the side and back of this setup you can post?
Before I mini-tubbed, the biggest tire I could fit on my '65 Coronet was a 275-60X15 on a 7" wheel with a 4" backspace. After minitubbing and moving the springs in, 12.5X29.5X15 Hoosiers.
I like that look aldal. Did you have to relocate the spring mount or is it still stock?