Well they may be 1/4 and they may be 5/16 but they are no bigger. I had to drill mine out to get 3/8" ID on my 383.

I’m going to use the factory pickups with the with -8AN all the way to the bowls. Approximately 600 hp, 509s.

Edit: the check valves will be removed.

Check out the Canton site. I'm surprised that the dyno data didn't show fuel consumption. From their formula your line isize is fine . 11/32" id is good for 60 gph and that's enough to support north of 700hp

A short run of 4' can be a size smaller than needed if it was say 10 or 15' on a car. Restriction increases with length of run just like in electrical wiring. so what they made power with on a dyno VS what is needed in an actual application can be significantly different.
That said -8 (1/2") "should" be more than adequate pick up to pump and pump to carb if your only running about 4' on each leg on a carbed application. So long as you keep the regulator close to the carb(s).
Also since fuel supply is essential I always tend to build a little overkill into the system. Over capacity will never hurt. Under capacity will hurt performance and can melt pistons in a hurry if it goes lean at WOT.