3/8 supply and 1/4 return just won't work, because the amount of fuel being sent back is too great for the line to handle. I tried it a long time ago. Sorry to say, the best (and more expensive) solution was a new Tanks Inc. tank with in-tank Walbro pump. These tanks come with already-installed 3/8" sized supply/return fittings that can be adapted to -6AN or equal.