Tire sizes for 69 Cougar

What would be good tires sizes for my 69 Cougar…I am looking for a set of Magnum 500’s, and when I get them I will need help to determine tire sizing. The 351 has been significantly upgraded, along with a 410 posi, so I’m thinking I need the widest 60 series that will comfortably fit in the rear. With that, 70’s in the front would be appropriate, but what size? Thanks in advance.

Here is a list of what others have put on their Cats.

WHEELS and TIRES.pdf (252 KB)

For whatever it’s worth, here’s what 215/70/15s and 255/60/15s look like on my '69. With correct backspacing, 275/60/15s will clear on the back, but they’re tight.

What back space do you have on the rear wheels with the 15X8 255/60R 15
Is it 4.25 or larger?

Here are pictures of both my 69 Cougars with the same Magnum 500 rim/tire combination. Fronts are 15x7" 4.25" back space w/ BF Goodrich Radial TA 235/60 15" tires. Rears are 15x8" 4.50" back space w/ BF Goodrich Radial TA 245/60 15". The fronts just clear the upper ball joint with no fender rubbing. The rears fit perfectly.