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My intention is to try to find out what and why the offset shackle was installed for. the front spring eye bracket is not offset so I dont know why there is a rear offset.



You may have to take a lot of measurements to try and figure this out exactly. Since it has been a drag racer in its previous life, I'd most likely say it was to increase tire clearance. As stated earlier, the E body springs are splayed to be wider at the rear. This type of install can reduce the available space for tires by a half an inch on the rear segment. Putting offset shackles in without a corresponding offset hanger could put rear segment of the spring more parallel to the front an allow a skosh more tire. However, this has also likely put the springs into a potential binding situation, unless the spring mounting pad location hole was also opened up a bit.

The splayed mounting arrangement does tend to reduce roll induced oversteer and stabilize the handling some. To restore this would require the offset front hanger while still allowing for wide tires. However, to do this with the stock E body housing will also require moving the spring mounting pad inboard the corresponding amount.

Another alternative is to use the offset bracket/shackles and drop in a B body housing which will fall right onto the springs without moving spring pads.

With the offset shackles, their mounting point into the frame horn should not have changed. The shackles are bent to create the offset. The mounting point does not change. Returning to straight shackles should allow them to bolt right back into the stock location.