Rear frame bent?

NorcalGolddust said
If I center axle to leaf springs, outer frame rail to outside rim is equal on both sides at approx 14"

This should tell you what you need to know- you missed something the first time with your measurement. If the frame was offset, you'd get unequal numbers for that measurement- think about it.

NorcalGolddust said
No. I cut off the old spring perches as I am using Doctor Diffs 1/2" offset shackles which put me at 42" center to center. I'm pretty sure I'm supposed to center the rear end to the leaf springs as that's how the old 7 1/4" rear end and leaf springs were. It's just the 1" frame offset to the pass. side is throwing me off
Spring perch widths (center-to-center)
A-BODY

'66-73= 43"

B-BODY

'62-'70= 44"

The perches need to be centered on the axle. Forget about where the springs and frame are. The springs can move on the bushings (some morons don't even move their perches when they install B-body rears on A-body's), the frame might not be straight, and pretty much every other body measurement has at least a 1/4" or more of tolerance from the factory. It's pretty common for the measurements not to be identical side to side if you're measuring to the wheel tubs or quarters. The axle housing has tighter tolerances on its measurements than anything on the body of the car.

8 3/4's are centered, equal length axle shafts. So the perch-to-housing end measurement should be the same on both sides. Your '69 B-body axle is 60 1/8" end to end. You need 42" c-c on the perches with 1/2" offset shackles. A little math and you get 9-1/16" for the center of the perch to the outside of the housing flange. Or knock off half the width of the perch for 7-13/16" from the outside of the perch to the end of the housing. Make the measurements on the axle right, the rest will work itself out. Or not, but at least the axle will be straight.

Keep in mind too that not all the backing plates for these cars have the same offset. If you don't know the history on that B-body rear, or if the brakes aren't complete or in working order, it's possible the backing plates don't match side to side. Another reason not to use the backing plates for measurements.