Webers don't care about a return fuel line; they should work fine with or without, tho' having a return line can help prevent vapor lock and stirs up fuel in the tank to help prevent debris from clogging the fuel pickup. You'd add a tee for the return line before any fuel pressure regulator you may have.

Some stock Mikuni carbs had two fuel inlets, so that's what the two outlet barbs on the stock mech pump are for, effectively a built-in tee. Webers only need one fuel supply, so you can either cap off the second outlet barb on the pump or just use it as a tee to the return line.