There's different ways to change the heights.
Stiffening the rear springs is not a good way. Doing so increases the tendency to oversteer. Oversteer will show up in the worst situations such as, downhills, at high speeds, and/or on slick surfaces.
Increasing the rear arch can be done cold or hot, but it will not likely hold up as long when done cold.
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 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. Bumper Heights
Not really sure the intended purpose of this document as these heights were never in service manuals, but its better than nothing when you're not sure. 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 uh slop.
That said, if
you carry significant extra weight in the trunk, stiffer rear springs will help. The additional stiffness will reduce the deflection from the extra load and the extra weight will mostly offset the oversteer tendency. if
you drag race, some additional arch, a thicker main spring, extra front clamps, and a strong second spring under the front eye will all help launch and reduce spring wrap. All but the arch will also help the spring survive longer.