"little "porcelain" type fittings" are roll over check valves. they stop fuel from leaking out of the fuel vent system when you turn you MJ upside down. they are mostly standerd shape and size on all fuel tanks. so you can find a bunch of them at the junk yard.
I do not think missing one will give you a fail becouse they both meet up at a T fitting befor the charcoal cannister The Idea of the system is to let the fuel vent off on warm days. the fuel vapers are stored in the charcoal cannister and sucked out when you start the motor. when you look at the charcoal cannister you should see the one small line coming from your tank and a bigger line going from the charcoal cannister to the air cleaner. every time I have seen them do the emissions test, all they did was check to make sure the charcoal cannister was there and the hoses going to it did not have any cracks.
the space between the tank and the skid is there to give the skid some room to absorbe bumps, so stuffing some thing in it would not help any