How to calculate tire size?

Well, I decided to go with steelies all around, kid likes the look.
I have 15x7 on the rear, bought from a member, made by Stockton with 5x4 centers and 255/60/15s.

I'd rather go 15s on the front, so I pull up the Wheel Vintiques and I'm thinking either 15x7 or 15x6 on the front.

I found examples of people running the 15x7 with both a 205/70/15 or a 215/60/15 on a 67-69 Barracuda.
None found with a 15x6.

Thoughts?

Also, I hear these wheels have a tendency to throw dog dish caps off. Anyone experience this?

Any feedback is appreciated.

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