You can run without muffler pressure, but I for one wouldn't advise it.
Adding a pressure tap is relatively easy. You just drill a proper size hole, tap the proper threads and screw in the tap. Then you connect a piece of fuel line from the pressure tap to the vent line on the fuel tank. If there are 3 lines going into the tank, the fill line will need to be plugged. This is not a big problem you can just stick a proper size bolt in the third line.