Welcome to Stovebolt, Red.
Is that 5/8 to 3/4" low as measured from the frame or from the ground? That will make a difference. The Factory Assembly Manual Section 1, Sheet 3.12 specifies 7/4" between cab floor and frame at the rear cab shackles. It should be the same on each side. the FAM also specifies 1.83" +.03"/-.00" frame to bottom of floor at the front mounts on Section 1 Sheet 3.09. Also should be the same side to side.
With that much difference, it sounds like your issue may be with the suspension.
You should head over to the Welcome Centre and introduce yourself and your truck. Tell us a little about yourself and your truck, such as how you acquired it, and post some photos as well (We love photos of old trucks
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