As long as the filler hoses and supply hoses are maintained then you shouldn't have any problems.
About the only time you could have a safety issue is if someone hits the cab hard enough to contact the tank, or if they hit right on the filler tube. A rollover could compromise the filler neck, but if the tank is properly strapped in it won't be tossed around the cab.
My brother rolled a 1971 F... and the tank did leak. He got nearly out of the pickup before the gasoline that had spilled out the filler tube ignited.
But because of the way the fire started on his truck it probably wouldn't have mattered if the tank had been under the truck. The gas would have spilled out anyway.
These old trucks have gone many miles with these tanks behind the seats with little problem. I still think they are safer inside the cab that stuck just in front of the rear bumper.
