rayra
Expedition Leader
There's a lot of room between the frame rails between those tanks, up high against the flatbed. Nothing in there but a couple cross members and the fuel tank switching valve / solenoid assembly, IIRC A large rectangular but shallow tank would fit in there well enough. Call it a yard square and 4-6" deep. There's about 230cu" to 1gal. That would be 20-30gal. I never got more than 15gal into those supposed 16gal tanks on my '85 C-10. So you could replace with a tank in that spot without losing much capacity. If you get one welded up from aluminum plate, you could conceivably gain even more than your stock fuel capacity.