I believe that flange is soldered in place, in addition to the two bolts. With the thin metal of the flange, it would be difficult to get a gasket to seal well. I think the bolts were there just to hold the neck in place while flowing solder into the joint. I can't get a close look at my tank, but I recall seeing it being soldered.
[on edit] Just to clarify: I didn't actually see it BEING soldered (I would have been 3 years old then, and they probably wouldn't have let me in the assembly plant), but I do remember seeing solder around the edges of the flange, when I removed it from the truck.
