[Captainfab]

I would position the trailing arm mounts to be the same distance from the backing plate on both sides. Keep in mind that your '87 10 bolt is 1.5" wider (.750" each side) than a '63-'70 truck 12 bolt. Then to center the differential side to side under the truck, you may need an adjustable panhard bar. Don't forget that you need to get the pinion angle correct before you weld those trailing arm mounts to the axle tubes.