72 dart rear tire size

What is biggest tire that fits ?
1972 dart stock quarter panel . Has old Mopar leaf spring in boarding kit ( belice those shackles moves them in 3/4” .

i have 15 x 8 4.5 back space small bolt pattern rallies .

I have 255/60/15 mt on now . Looks like I can go way bigger .

I’d like to put 275/60/15 MTs like my duster and challenger have .

not sure if that will fit . Anyone know ?
If not I’ll settle for 255/70/15 cooper cobras .