The cutoff is in case of a roll over it cuts the fuel pump off.

Return line is to gas tank.

I run a 1st gen RX-7

I have a fuel cell and deleted the cutoff switch. I use a return line.

There is some discussion about the need of a return line in a race car. I think it helps keep constant pressure at the carb. I run 21/2 lbs pressure. I use a 1/2" fuel line to the carb and a 3/8 return to the cell. Never had a problem.

Drive well.