you can only decide how wide the diff is going to be once you know what size wheels you are going to run plus the backspace... If you keep the leafs in dse kit and relocate shocks then it will allow for a deeper dish wheel than stock obviously... Eliminate the leafs for a 4 link and again it changes and allows a bit more offset in the rim .. And a bit more of a wider tire.. I have the dse with the leafs... 3rd gen.. 295 no problems.. 315 i think is all it will take without rolling ...i need to buy a new rear end for my 1970 nova and will be using dse tubs, and either dse quaralink or ride-tech bolt on 4 link. Wondering if it would be better to get a slightly narrow diff to while i’m at it?