Here are some common issues with behind the seat tanks
The thing rolls and the fuel goes out the cap side down the side of the truck. Possible fire
Fix;
Sealed cap and vent lines running to evap canister.
The cab smells like gas as tank vents
Fix;
Sealed cap and vent lines going to a evap canister under the hood then pulled into engine.
The truck is hit in the side or something thrown through rear of cab and tank ruptures
Fix
Flatbed has a roll bar that extends to the outside of the cab attached to the frame protecting tank and filer neck from this exact situation.
Filler neck rubber cracked or sending unit seal worn
Fix
Maintenance. Periodic checks at oil changes. I will make my fill so that 3 bolts hold it in allowing easy replacement of the hose without moving tank.
Derbies within tank settle to bottom of tank and block fuel line.
Fix
Return line located above tank drain to "turn up" any derbies so that smaller amounts can be filtered before inline pump.
Water,snow, ice form around filler neck.
Fix;
Gas cap inset using hinged gas cap cover that matches close to body profile with spring to keep closed. Weatherstripping around cover should avoid old issues of brushing off snow.
Benefits of in cab tank.
Less tank rust as it is inside the cab out of the salt;
Here is my current passenger tank I am not going back to this setup.
Less water inside tank.
Tank is inside the cab and insulated better then one with a foot of snow
sitting on top/under it. Sealed gas cap prevents water from entering better then old vented gas cap
Tank is protected from contact with ground or being crushed against frame.
Tank will not receive salt spray like it will if located behind axle.
Mostly vertical tank has less chance of starvation then a mostly horizontal tank.
Tank is out of the way allowing easier maintenance.
Easy to replace sending unit and inline pump means once secured the tank should not need to be removed for years.
Drivetrain options.
Route my exhaust any way I want. Run longer springs changing wheelbase if I want. Shorten the frame behind spring perches If I want.
With the exhaust I want to build some fender well headers to a flexible section and v bands. It should allow me to unbolt and "pull out/down" the headers slightly making maintenance and engine removal easy. I don't have it all figured out yet. That is down the line in this. But I believe I can route the primaries under the frame protected by bump stops and follow the frame back then run to a flexible section to under the cab on the outside of the frame without much issue.