Plumbing to gas tank

[c10beezer]

Can someone help me out here? I have a 68 c10 that I bought in pieces (someone elses project). I am running my fuel lines to the stock tank behind the seat. On the sending unit I have two lines 3/8 and I am guessing 1/4. The 3/8 would be the feed but would the return be that small? My carb return is a 3/8 line. My other question is on the passenger side of the tank there is a 3/8 fitting what is that for? Thanks for your help.