How much weight can you safely put in the back of a '68 El Camino? I have an SS with conventional shocks/springs in back (no air). Once I carried 900 lbs. of landscaping rock home from Menards and it was noticably loaded down, but not to the point of being dangerous or undrivable. (Boy did it HOOK though!) Just curious what the payload capacity was, like you hear of 1/2 ton trucks, 1 ton trucks, etc. . .