Ricky;
This chart should help.
In my 1948 Plymouth P-15 I installed an 8 ¼” differential. I rebuilt the differential and installed 3.55/1 gears. It was almost a bolt up. I had to weld a couple of plates on the existing spring perches in order for the spring centering pins to line up correctly. This in effect lowered the car by ¼” and I did not want to do that so to compensate I made new rear spring shackles that are 1” longer.
I was able to use my original lower plate but I had to massage the holes a bit and put a slight angle on the U bolts.
I also replaced the shock mounting bolts as the old ones were trashed.
The installation of the differential was one of the easiest parts of building my car. Hope this information will be helpful.
Don Coatney
