Can the fuel rail shrader valve be used as the return to the Fpr then step up to -6an from the regulator to the tank? Or is it to small. I think the shrader valve is -4an or should I just get different rails or put the regulator before the rail?
Truck in question is a 08

 

The shrader valve is to small. Are you talking about running an aftermarket regulator? If so and using factory rails just run the supply line to one side port on the fpr and then run you a hose to the rails from the other side port and use the bottom port for you return , you'll definitely have to run a new line to the tank.