I'm not sure about the legal situation here in FL. The local truck store (tops, toolboxes, bumpers and guards, etc.) said the law prohibited in in-the-bed gasoline tanks (like a toolbox tank) b/c gas explodes whereas diesel burns. But he indicated it was federal, not state law. On the other hand, plenty of websites seem to carry in-the-bed gasoline tanks, so it would not seem a federal issue. I thought about asking the local Highway Patrol, but I'm considering the danger of an in-the-bed tank, too. With the right accident the whole truck and passengers would be a molotov ****tail.
A lot of guys put the aux. tank in the bed and just use gravity flow instead of a pump. That way the fuel gauge always reads full until the aux is empty and the main tank starts to empty. But even the cheapest aux is around $400.
So far the best bet is a replacement 46-gal tank for $800. But this truck is a 2000 with about 112K on it. Putting in the replacement tank is a gift to the next owner, since I will never get my money out of it. I carry a 5-gal spare on trips for emergencies, but you still have to pull over every couple hundred miles.