There were a number of different springs available for each model depending on engine and other options. If you are not increasing the front spring rates (t-bars) I'd be very hesitant to increase the rear spring rates. I don't have books for 1970 and newer, but with a little digging (maybe asking over in restoration or the Hamtramck-Historical website) you can find out what the factory suspension packages consisted of, including the Heavy Duty options.

Stanley Parts has a number of leaf options, some even cross ref. to factory numbers. They're based in Harrisburg. See here (Allentown retailer's website) for choices:
http://www.stengelbros.net/Dodge-Coronet-Charger-and-Super-Bee-Leaf-Springs_c_465.html

Engineers generally aim for 1 million cycles on a spring. Leafs and coils will then flatten/ compress/ and lose some rate. So yours very well may be lower than new. Without other info., I assume leafs will start to go downhill around 75-90,000 miles. Shorter life if overloaded, twisted (eg drag race start), or otherwise pressed to their limit.