My 68 442 w/400 cu engine needs a new sending unit & fuel pump. The present units have two lines, one for the fuel from the tank to the pump and a return line from the pump to the sending unit. I know there is also a sending unit and fuel pumpfor the 68 with only a single line. What is the purpose of the two lines and can I use a single line system with my 400 engine? The carb is non-original -- an Edelbrock 650 CFM.