Measuring for four link rear suspension

So the plan is to remove everything from the frame, from the cab back. Build a new crossmember at the very rear of frame to fit fuel tank between frame rails.. Repair and strengthen the frame where someone shortened the frame and just butt welded it together. 

Box the frame in key spots, replace the GM 10 bolt with a Ford 9″. The differential currently setting 1.5″ to far back. So I will center differential and shorten differential 1″ each side. It then gets all the four link brackets welded into place. Setup and adjustment to complete the install.