ive been running ag tires on my blazer there 13.5 x 44 in tall. and been breaking my rear end. so i got a 3/4 ton rear now and want to build rims for them . i got the wheels now there 12 in wide and i want to cut the insides out and put in my new 8 lug pattern but dont know what backspacing i need? i got a 6 in lift on it.