Factory rake of a 1970 Dart Swinger?

Alan, That's awsome.
I don't recall if I'd seen that before.
The only rear height info I've seen from the factory is at Hamtramck Historical.
The 1970 Hamtramck Registry "Chrysler Passenger Car Bumper Heights 1965 thru 1976" Page 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 specs, not so much suspension setup info.

From a suspension perspective a flat spring when typically loaded is basicly correct. This is how Chrysler designed 'em, and why there are so many different spring packs to match the various weights and use. Chrysler explains it in the in this booklet about handling.
An exception were the cars intended for drag racing which came with the super stock springs. Arch is one way to increase anti-squat.