switching 14" axels to 15"

i have a 73 duster with 489 8.75 3.55 suregrip and would like to get bigger tire on for better traction, i have 14" tires now and would like to maybe put 15" tires on back, can i just switch axels to fifteen inch or is there another solution to my problem, if i stay with 14" what is widest and tallest tire i can put on without hitting springs, any ideas are welcomed? thanks