This is why production springs have very little arch, for toe control.

Drag race springs (without traction aids) generally want stiffer front segments, a higher front eye, and bias for launch control.

Exactly.
Stock springs were designed to be flat under average intended use.

The shocks only have about 1.5 and 2.5" of rod exposed.

Exactly.
Stock springs were designed to be flat under average intended use. That's why there were so many different spring packs.

reposting from another thread:
Increasing the rear arch can improve launch on the drag strip if properly combined with sticky tires. It is also used sometimes by circle track cars in combination with other changes to improve coming off the corners.
On a production car, arch can have negative handling effects.
Flat is basicly correct. This is how Chrysler designed 'em. Chrysler explains it in the in this booklet about handling.

Trying to find the original rear ride height is difficult.
The only rear height info I've seen from the factory is at Hamtramck Historical.
http://www.hamtramck-historical.com/PassengerCarBumperHeights-01.shtml Note that on page two that these are for fully loaded (5 passengers, full fuel) but does not say what options. Also the bumpers are somewhat adjustable and the body has some slop. So these are visual body related, not so much suspension setup info.


If you will be taking it to the track, then the arch may be helpful.

Adding helper springs is only good for specific loaded situations. Otherwise its stiffening the rear roll rate and can lead to oversteer when your not going to appreciate it.

It rear steers because “toe” has changed through suspension travel - toe relative to the centerline of the car.

On a hard turn one rear tire toes in the other out - relative to the centerline of the car.