After Market Spring/Height ID???

[67WrenchHead]

This may be posted somewhre already, but I couldnt find it. I've got a '71 Step Side, looks to be lowerd about 5" but not sure. I want to raise the back end, back up about 2". In Theory, if this is a 5" drop, a 3" drop spring should be perfect?? Or air bag and adjust as I go? On my Chevelle, the Factory Assembly Guide showed where to measure and had different specs for different option packages. Is there anything like this for the trucks?

Thanks,