How are you measuring? Is that total travel, just up travel or down travel? Provide some more details please. Sounds like your aftermarket springs may be waaay too stiff. Stock springs work great for SOA rigs.
From the previous owner who responded today, they were from a company called Benz Springs in Portland. We measured from point to point. Raised the body up and left the wheels to hang to that measurement. In a sitting position 16" from point to point and 18.5" extended. Might be good for a track cruiser but I might age my back if it ever makes it to the trail.