Good advice. Wait till you have the body on and all the weight on the suspension. With the weight on the suspension set your pinion angle and tack the perches in position, then remove the rear and weld them on. Its the best way to set your pinion angle. AND use the second example in the illustration as a guide. The output shaft of the trans and the pinion need to be as close to parallel as possible. Also the trans output shaft should be a few degrees down which means your pinion will need to be about the same angle in the opposite direction in order to be parallel.
"The only place you'll find a helping hand is at the end of your own wrist" - Joe "Dr. Olds" Mondello