priobe,
I have the same gas tank. If I try to fill it full force from the gas pump it will not be anywhere near full when the pump stops automatically. It has always been cranky and slow. If I fill it slowly it will go all the way to full. It takes a few minutes of hand-squeezing the pump handle lightly. It has also taught me that the second time it shuts off while I'm doing this, I'd better quit unless I want gasoline on the fender.
However....
You knew there was a however
When the tank is really really full it will overflow. Not right when I fill it, but later after the gas warms up and expands. Unless I drive enough to burn off a gallon or two before I park it.
There's another observation too. If I fill the tank quite full, it will overflow when I take a corner. This can be detected by looking at the fender on the gas tank side after a run, where a residue and some road dust will be clinging to the area from the filler cap all the way down to the bottom rear of the fender below the tail lights.
The cornering-related overflow happens occasionally on the street and very frequently on the track.
To reduce/eliminate the cornering overflow I installed a check valve in the fill line. I found the valve somewhere (?) - sorry forgot where - a guy makes them to fit the '57 Chevy wagons which have this problem only worse. The valve is gravity operated and made of stainless and happens to be the correct diameter for the Cobra fill hose. It works fine!
I have the feeling I may not have answered the question
If not, ask away!
Tom