Johns logic is simple:
16,000 lb GVW
-4,500 lb for cab/chassis
-7,000 lb for 1,000 gallons of fuel (approximately)
=4,500 lb for the tank body

It can't be light, and removing it I can imagine will be difficult... Hire a wrecker for the whole thing

Looks like an interesting project,

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-