What size fuel line is absolutely required?

Folks, I am installing a 512 stroker in a 69 Dart abd I am tired of doing things twice.

I will eventually have either a Sniper or FAST system on and the goal is 600-700 HP. I would not think that's at all unreasonable for a mostly street car.

The question is will -8 line be required, or will -6 be adequate to not create lean cycles?

Also, will I be able to run a carb off an efi pump with a good regulator and the return line plugged?

Too many mistakes have cost me money so I may have to run my 950HP for a while.