Hi Jeff welcome to the forum. The axle stops on the grand that mount to the underside of the rails only come on that truck and are used if you use the rear fenders. They are supposed to keep the tires from getting into the fenders. I believe the thread pitch on the 3mm is .5. Hopefully someone else will jump on here and confirm that because I'm not in my shop at the moment. As far as the suspension location, in the 1:1 world that would be up to the customer and what they are going to use it for. So in the modeling world here put it where it looks good to you since its your truck. I would suggest that you don't drill holes and mount the rear suspension until you have the bed/body built and on the chassis. The body will change the way the truck looks so it's best to wait or you'll end up with a frame full of holes you don't need.
Kent