There are a lot of drag racers running larger cc 's and dual 48IDAs on the stock fuel lines with no fuel delivery issues, though a new 3/8" would make it easier to plumb in the fuel pump and regulator with AN fittings. If you are planning to run a turbo or FI in the future, you may want to consider running dual 3/8" lines; separate lines for feed and return.
Also, you don't need to "weld a AN-6 fitting to the fuel tank outlet". You can get a -6 AN fuels tank adapter (see below) through most of the major online air-cooled VW parts sources. Make sure to run a quality fuel filter between the tank and the fuel pump.
On the fuel regulator, a Holley or Mallory dual-output "dead head" regulator with a gauge port will work just fine. You can use it as your "T" to feed both carbs. Avoid the dial-type regulators. Set the regulator for a max fuel pressure of 2.5-3 psi under load. As with above, if you plan to run a turbo or FI in the future, you may want to consider running a regulator with overflow return capability.
