What size fuel line is absolutely required?

Yes, a 3/8 line (#6) is plenty big for EFI. If someone wants to do the math it isn't too hard. 700 hp needs 350 lbs/hour of fuel which is basically 60 gallons per hour. Look up a flow rate calculator on line for 60 gph in a 3/8 tube and you'll see that it is well below the critical velocity. There are flow rate charts on the web that show you what line size you need for how many GPH. The tough math has all been figured out for you so you just have to look up the number on a chart.