Early Classic Enterprises (ECE) has a section called Customer Trucks. There's a guy that has hauled 1550 pounds with his lowered 66. He didn't say what the rear drop coil height was but there are pics of it empty and loaded.
https://www.earlyclassic.com/node/33
My truck isn't on the road yet but this is why I decided to go with ECE springs and spindles.
No, I get no compensation from this statement … LoL