Wheel Size for 1968 Barracuda

Hello everyone.

1968 Plymouth Barracuda Coupe

Small Bolt Pattern (5x4)

Unsure of backspacing

No modifications have been made (Stock)

I have a pair of 15inch oem Chrysler steelies they are 15x6 for my small bolt pattern which I am going to mount in the front with 235/60R15

In the back I plan on putting 255/60R15 but I have not purchased the wheels yet. Before I do I’d like to get some help/advice/answers/suggestions.

So my questions are:

What size wheel will I need to fit that 255 in the back?

Do I need 15x7 or 15x8? (Remember this is SBP)

Does anyone have similar wheels and tires on their 67-69 Barracuda?