IMO.....Original installation was 5/16", U code engine, no vapor separator, The six barrel engine, V code, would be 3/8" with fuel vapor separator and a separate 1/4" vapor return line back to the tank. Just my opinion......Was wondering the correct fuel line size and setup on a 69 440 HP? 5/16 or 3/8". Did the 70 HP have the 3/8"? Would the 3/8" be better? Thanks