Did 70 EC and Chevelles come with the same stock rear springs? I bought some aftermarket moog 70 rears that said EC, but were the same model number as the Coupe.

Originally no, but aftermarket manufacturers usually only make a few spring rates and make them availible for many models. The Chevelle springs will physically fit an El Camino and vise-versa however they will not be the correct rate for one or the other, or possibly both. Your best advise when picking springs is to spend a few hours getting aquainted with the Moog spring book and the spring charts to find the exact spring to fit your needs based on what it calls for, for your vehicle and what you want out of the spring. I did this for my 1972 El Camino and am very happy with my selection.

Originally, the springs were computer selected based on the options the vehicle was scheduled to recieve on the assembly line. (each option was assigned a certain weight, the weights were added up and added to the base weight of the vehicle and springs were choosen accordingly)

Thanks for the info. I have already installed the springs, but waiting on axels, so I wont know how the ride height looks for a while.