1970 duster project

it depends are you going with stock wheel wells you can start with 5 1/2" on a ten " wheel my car has a spring relocation kit and mini tubs so i went with a 6.80 and it was too much so i used a 1.25" spacer kit because i later found out that the housing was narrowed 4" but a stock rear end on my charger and gtx was 5 1/2 " fits great looks good rjsea would be some one good to ask also