I believe I can help you!
After installing a larger fuel pump I ran into a BUNCH of issues. My gauge was always off one dot. Whenever I filled my car up gas would come pouring out of the bottom of my car. And then I had an issue where my car would stall around 1/4 tank.
You're going to have to physically check the fuel pump assembly. Two main issues to look for would be the float isn't settling in the tank properly (this would cause your readings to be off like they are -problem 1 of mine). And make sure the T fitting is securely pressed into the regulator outlet.
VERY IMPORTANT: MAKE SURE THE BLUE O-RING THAT SEALS YOUR FUEL PUMP ASSEMBLY TO THE TANK DOES NOT EXPAND OR GET DAMAGED. IF IT DOES NOT SEAL PROPERLY YOU WILL HAVE GAS LEAK OUT OF YOUR CAR UPON FILLING THE TANK....
Here is a site I used to install my fuel pump:
CJM Tech Site: A First Look: The 370z Fuel Pump Assembly
I called CJM up when I was having problems and they answered all my questions. Very nice people.
Hope this helps, let me know what happens!