common now thats not bad advice, Sure there are not many options in 14s and 15s and maybe custom made tires are not the best option for something that gets driven often(I honestly know nothing about above diamandback tires)
OP was looking for advice on tire sizes in a 15" they already purchased, why cant that simply be what this thread is about? Not everyone likes the larger wheels on their cars and honestly they can look out of place on a stock car. I would not be going 17s without lowering and upgrading suspension items on my car to match. On a stock cougar I think a 15 and a bfg radial TA is a pretty damn fine look and like was mentioned kinda period correct which appeals to many with 60s era cars.
I can not recommend a proper tire size for the 15s except to reference the tire and wheel thread
I do like those legendary wheels and wished they still made that style in a 17 and I may have chosen that wheel for my car