There's a 48gallon on my '73 K5. It was there when I bought the truck from a church in Arizona, who was the original owner. They said they had that tank put on in the 70's. I'm currently in the middle of frame-offing my truck, all fibergalss body, etc...etc..etc..., I had the tank Re-nu'd (it wasn't leaking, but while it was off I paid the $100), it has a skid-plate built into it...
I wish I know who made it, it could be a custom job