Another biggest tire question

I want to put a wider and slightly taller rear tire on my 69 Coronet, and I'm not sure what I can run. I'm wanting to run a larger tire on my 15x7 500's. Every thread and bit of info I can find refers to x8, x9 or x10 rims, but what the widest I can run on a x7 rim. I currently have 215/65/15's on the car and plan on leaving them on the front. Will a 255/60r15 fit in the back?

Also anyone have pics of x7 rims with a wider tire? Preferably on a Dodge, all I ever see is Plymouths and I've gotta believe the wheel wells are different enough…

Pic for reference

Ended up going with the 255/60r15' in the back, also got 235/60r15's up front. There is room to go ore obviously, but I'm not sure I'd be able to run a wider tire on the x7 rim…

If you get on tire rack they will show what manufacturers recommend on safe rim widths for any particular tire. Most large tire shops will not go outside of these recommendations do to liability issues.