So the stock steel fuel lines are large enough for the EFI upgrade ? Are they 3/8 " ?
I’m going about it a little different. The stock lines will only carry the fuel being fed by the stock mechanical pump. The mechanical pump brings low pressure fuel to the surge tank, then the stock return line feeds excess fuel and any vapors to the tank. The same as the stock carb‘d system would do.
The surge tank has 2 additional fittings, one is the high pressure feed from the submerged fuel pump feeding the throttle body. The other is the high pressure return from the pressure regulator built into the throttle body. Once the high pressure fuel return dumps into the surge tank it is back to low pressure awaiting pickup by the submerged pump again.
Think of it as 2 separate loops of fuel tied together at the surge tank.
All of the lines that are high pressure are -6, same as the fittings from Holley.