3/8 vs 5/16 fuel lines

A buddy of mine gave me a new 25' coil of metal 3/8" fuel line. I will use it on both cars just because I have it, and both cars will need lines put under them at some point. My 408 might actually need it, but sonny's warmed over 318 wont. But that's ok. It will be in there if he ever does need it down the road.