The only big issue I would be concerned about is some water buildup in the tank but if you had a interior flush bottom bung that should be eliminated. The second smaller issue is how to prevent overfilling/overflow of lower tank. I am sure both can be solved easily. I know this has been covered in the past do a search and I am sure you will find some good posts. I have a tank in my truck for plane fuel with simple 12 volt pump, no issues in 13 years. DENNY
What my thoughts are, is to weld a 1/2NPT bung on the fill pipe and use a rollover valve. I dont want to cut the fill hose and use one of the "kits" most use to do the install
One of my questions is how do I keep the fill pipe from over filling? This is one of my biggest concerns. I just dont want any spills or leaks.
I know you want a gravity fed system but I'd encourage you to consider a transfer pump. I've been using a 13gpm pump for years on the 91-gallon RDS in the bed of my F250. I run all the fuel through sediment and water-removal filters, so I know I'm putting clean fuel in the OEM tanks. I also put a long-enough hose on the pump to reach other diesel equipment, such as tractors etc. It makes the system very versatile.