Don't know what system you're upgrading to, but my Sniper EFI says 3/8 line. I went with the Sniper EFI tank for the whole package, so since I had to run a return line anyway, I changed out the 50 year old feed line too. If you need to use any flex line in the system (I did so I could run an in-line fuel filter and the AN fittings are so much easier to work with), make sure it has carbon infused PTFE lining. It will resist the degradation caused by ethanol and the static electricity from the higher flow rates for EFI. FYI: If you're working on getting rid of the fuel smell, you might consider adding a vapor recovery system and a non-venting cap if yours was a vented one. I talked to a guy from II Much about their venting system - he was a wealth of information. Got their system; haven't gotten to that point on the build yet, but I have what I need for what I think will be a destinkafied Mustang.
302, 351 World heads, Edelbrock 302 Performer intake, Holley Sniper EFI, Hedman full length headers, custom side exhaust, Shelby style hood
Dakota Digital Dash, Classic Air A/C, Borgeson P/S, Ididit tilt column, modified interior (2 seater), full console w/touch screen