go by your wheelbase. mark exact point of centerline of front wheels, run the length of the wheelbase and transfer the exact endpoint to the center of you hub on your rear axle.
also, the mounting plate for the leaf springs has the hole in the center that can be used for reference when use with the bumpstop center. they're made to match up, i believe.
good luck!
PS: when you line up that rearend, make sure you have the frame at ride height. if its out of position, your driveshaft measurement can be off later. oh, and dont forget to set the pinion angle on that rear BEFORE you put those brackets on!!
