When I replaced the sending unit on my '50, I just capped the "new" outlet in the sending unit, and still use the bottom outlet.
If the tank is clean, I can't see it mattering.
The only down side to the bottom outlet, is that if your fuel line, or hose, or pump break or rupture, all the fuel in the tank will drain. If you are working on the fuel pump or lines, you want to make sure your shut-off actually works. Ask me how I know this! Hahahaha
