You can get Dot cert gas transfer tanks. Look at JME or RDS tanks.
Not sure about larger belly tanks for gas.
I'd be comfortable plumbing in a gravity fill with a valve to the filler hose. Not sure if it's legal though.
Based on your last 2 threads about your truck, you're now asking about spending as much as the truck is worth potentially in upgrades to a 14 year old pickup that has a low resale.
If you're on that strict of a budget, save your $ and run what you have. It's a pretty reliable setup if it's been well maintained.
Otherwise pool up the cash you're thinking about dropping on all these mods, sell the 5.4 and get something more suited to your needs.
You'll never get 2cents out of the upgrades you're planning when you sell it.