There are far too many old wives tales, myths and just plain false information when it comes to leaf spring perches and their proper installation. I am going to show the proper installation and the proper Ford factory pinion angle on any leaf spring equipped Ford with an engine that produces up to about 400 horsepower. After that amount of horsepower, the resulting pinion angle will be less that the one I show. The pinion angle that I show is the same for any Falcon/Comet, Ranchero, Fairlane, Mustang, Maverick, Torino, Granada etc. The leaf spring perch center to center distance will change depending on which car you have. I have worked on hundreds of Ford 8" and 9" rear end housings, and I have measured the pinion angle on dozens of them to arrive at the angle that I set up housings with.
While you can weld them in the car with everything clamped in place, I use a far simpler method that allows me to install the perches on a bare housing on a bench. I use a simple "C" channel jig to install leaf spring perches. The channel has a pair of 18" long x 3/8" wide slots milled in it to allow leaf spring perches to be installed at any center to center distance. In order for the bottom of the rear end housing to clear the jig, I use a couple of 2" x 4" steel blocks for the leaf spring perches to sit on. The steel blocks have an allen head bolt that the spring perch loctaes on, and a stud on the bottom that will bolt the the jig.
With the steel blocks set at the 42 3/4" center to center Mustang/Falcon spring perch distance, the housing is set on top of the pads. A measurement is taken from the edge of the spring perch on each side to the edge of the housing, and adjusted until the measurement is exactly the same on both sides of the housing:
The spring perch jig is sitting exactly level on the bench that it is clamped to. A magnetic angle finder is placed on the centersection mounting surface, and the housing is rotated up until the angle finder reads 5 degrees up. 5 degrees up is the Ford factory angle measured with the spring perches perfectly level. The pinion angle will change once the housing is bolted in the car, but this is the method and angle to use when welding them on using a jig on a bench:
Measure twice, weld once!!!! Go back and double check the side to side measurements and the pinion angle a couple of more times. With everything where it needs to be, tack weld the perches in four places:
I now weld the perches halfway on one side of the perch to prevent warping of the housing tube. I flip the housing over and weld halfway on the opposite perch. I continue to alternate welding the perches from side to side until they are both completely welded:
The housing is now ready to be bolted in the car: