Not sure about the 3/8" on 440 cars. Thought that was a hemi thing, but I could be wrong as I don't have any factory 440 cars.
Summit sounds like they're being pretty pessimistic, I know of cars well over 400 hp that didn't have fuel issues with 5/16" lines. I replaced the lines on my cars because of their condition. The factory stuff wasn't stainless, it doesn't last forever. It also gets kinked, dented, etc. The line on my Challenger had been hit with a jack at some point, for example. So it just depends. If the lines are in good condition they're not a "must replace" deal IMO, just blow them out and off you go. But, if I was going to replace them I would probably just use 3/8", it doesn't cost much more, only takes a few more minutes to install (a little harder to unfold and route), but then you know that's taken care of and won't be an issue. And same with the stainless. A little more money, but I know I'm not gonna have to do it again in my lifetime.
