I have a 71 road runner I am working on and have a couple of questions on changing the rear axle to an 68-70 b body 8.75 rear axle housing. I was told to just move the spring perches out so they are in the same place as on the original location of the wider housing. This would allow a much wider tire to fit within the wheel wells with a proper backspaced rim. My question is the spring perches will be close to the housing ends. It appears that it will fit without any backing plate interference. Has anyone on here done this and is all that needs done is to move the spring perches without using a spring relocation kit.
I haven't done it, but have considered the same swap.
Looks to me like the perches on the old, narrower rear end would each need to be moved out approximately 1 3/4 inches. So the old rear perches would be at 47.3 width out from its original 44.0.
You are right about the offset though - because all the width you gain will be outside of the hub if you don't relocate the springs.
Body line ..... ..... Track ..... Perches
------------- ---------- -----------
A-body, 65-72 ..... 55.6 ..... 43.0
B-body, 62-70 ..... 59.2 ..... 44.0
B-body, 71-72 ..... 62.0 ..... 47.3
B-body, 71-72 ..... 63.4 ..... 47.3 station wagon
C-body, 64-72 ..... 63.4 ..... 47.3
D-body, 64-72 ..... 63.4 ..... 47.3
E-body, 70-74 ..... 60.7 ..... 46.0
