I bought the 3/8 front to back line on the 67 I have now. I got the pump with the return line fttting from the pump. The pump is designed for 5/16 input but the 3/8 hose clamps on fine with no issues. I used the steel line from the pump to carb that allowed the use of an in-line filter. Looks stock but I think if the return was on the pump the steel line to carb had no filter (just the quadrajet inlet filter). Since you already have the 5/16 line from the tank I would go ahead and use it. You should be fine to connect the 3/8 sending unit to a 5/16 line without a problem. By all means you should use a return line, especially with todays available gas that practically boils at room temperature!
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