so your using small ball joint A-arms with large ball joint spindles and Dr Diff adapters. you say you can't get the cotter pin in, which is fine, but before that point there are two things that need to be done. the ball joint needs to be screwed into the arm all the way, is it? then make sure the boot is seated all the way against the bottom of the arm. after the spindle is in place the castle nut needs to be torqued. if the hole in the ball joint is covered, tighten nut until you can get the pin in.
what spindles are you using? if they come in the kit, can they tell you what they are?
