Any given spring can have a different spring rate and load capacity.
Some springs are constant rate meaning if you add 400lbs the spring will compress 1" another 400lbs will ompress it another inch.
A progressive rate spring ex. 400lbs compress it 1" another 600lbs compress it another 1" another 800lbs will compress it one more inch.
For OEM I believe the Big Block spring is the same as the heavy duty spring...