rallye rim tire selection

hi im sure its been ask before in another post but my search has come up empty in looking i have not been able to find a definite yes or no to my question

my question is if i have a dead stock 73 duster body with stock shackles and springs and a stock a body 8 3/4 what is the largest tire size i can run on a Mopar 15x7 rallye rim i have in my mind 255/60/15 or 275/60/15 i think the 275 might be pushing it so im leaning to the 255 also i think the 275 might be to wide for my rim (opinions are welcome on the 255 vs 275 issue also if they will fit)

also my 8 3/4 has had Yukon large bolt axles and 11" drums put on it though i doubt this would make any real difference another note i had to use the deeper chrome beauty rings if this gives a hint to what offset i have