Stock pump is fine...
Stock line sizes are fine...
Like Larry suggests, you split the line out of the fuel pump with a fuel block or log as they are sometimes called. A cheap and easy way would be to just use a brass "T" fitting from the hardware/parts store. Plenty of fancy finned aluminum choices out there also.
Usually I use 3/8" tube from the tank to the pump, and 5/16" tube at the carbs. You could use either 3/8 or 5/16 between the pump and the splitter. This setup doesn't have as much fuel demand as you might think, so overall volume isn't really something I'd worry about.
I've seen brake lines used, copper tubing with compression fittings, fuel grade hose with barbed fittings... They all will work if installed properly using some "common sense".
If you use hard line all the way up to the fuel pump, make sure it isn't "tied down" too close to the pump, or you may have a problem with motor torque and frame flex.
It's not recommened to use one of those cheap glass inline fuel filters in case of breakage. Replace the glass with a piece of tubing or O'brien Truckers makes a nice finned aluminum insert. Other filters are also available elsewhere...
Make sure your carbs are identical, and that you don't plug up the vac hole at the bottom of the carbs.
