What size fuel line is absolutely required?

Depends on the pressure. If you are eventually going to go with EFI then plumb it now for EFI and be done with it. Put a high pressure pump inside the fuel tank and then use #6 lines for both feed and return. You could probably use a #4 line for return but dual #6 lines will work.

Dual #6 lines will work just fine with EFI since you are operating at higher pressure. (If you don't believe then go to your local Dodge dealer and look at the fuel lines on a Hellcat. Yep, #6)

The trick will be if you stick with the carb. In that case you'll need a regulator which can handle the high pressure from the EFI pump and regulate it down to carb pressure. You'll end up not needing that regulator anymore once you switch to EFI. The Sniper wants 58 psi so you should put a pump in the tank that can handle 60 gallons or 300 lbs per hour at 60 psi. That should be big enough for 600 hp on EFI.