I have a chevy engine, the engine that was in it before was a 360 amc and there was a return line going back to the tank.Can I just block off this line?Also which one is the return line?

i blocked it off on my 258 after that and the gas tank started leaking.......been running a boat tank in the back since then with one line from tha tank.

the return line should be 1/4 in. while the feed line whould be 5/16th in.