read this, it has everything you need to know about drivelines, transmissions, pinions, there angles, relationships and how to set them.
http://www2.dana.com/pdf/J3311-1-DSSP.pdf
You should have the weight on the suspension when you set your pinion angle. Tire size doesnt make a difference, but a difference in tire size from front to back does if it effect the rake of your vehicle. The change in rake has can effect the angle that your engine should operate at, leaning 3-4 deg back (output shaft down) to help with bearing wear. from there all of the information you need to know is in the pdf reffed to above. Your 4 link should have allot of adjustment to get the perfect pinion angle when all bolted up, just do your best to get it as close as you can when you go to burn it in during mock up.