Ricky;
This chart should help.
![](http://img22.photobucket.com/albums/v65/DonCoatney/Don%20Coatney/mopar_axle_sizes.jpg)
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.
![](http://img22.photobucket.com/albums/v65/DonCoatney/31.jpg)
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.
![](http://img22.photobucket.com/albums/v65/DonCoatney/Ubolts.jpg)
I also replaced the shock mounting bolts as the old ones were trashed.
![](http://img22.photobucket.com/albums/v65/DonCoatney/18.jpg)
The installation of the differential was one of the easiest parts of building my car. Hope this information will be helpful.
Don Coatney