I'm thinking (1) 5/16 (2) 3/8 & (3) the special one with an extra nipple for a fuel return (which can be capped if not useing a return). senders are the worst of the worst for improper readings sent up to the dash gas gauge so definitely do your homework on this one. there have been several good threads on this in the last several weeks, see if you can retrieve em or someone my post a good company to do business with. Have your customer slip a 5/16 and a 3/8 open end wrench on the short metal nipple on his sender to see what he has now. OE would not have the return nipple so that elims that one of the three. But the problem is getting one that electrically reads right.