Fuel Gauge Question

So ever since I reassembled my Dart, which included a new fuel tank, lines and sending unit (everything in the fuel system), the fuel gauge only reads 3/4 full when I have filled the tank all the way up (nearly to overflowing). My question is, when the gauge is down almost to "Empty (E)" (I have not passed the E line yet) and I fill it up, it only takes 9-10 gallons. So I am safe to keep driving for awhile even after it hits E since the car presumably still has 8-9 gallons in the tank, or am I risking running out of fuel (or at least reaching a point where it does not pick up any more fuel)? Would appreciate any thoughts you guys might have on the subject.
You have an aftermarket sending unit. All aftermarket sending units are linear so they don't read correctly. See this thread on the drawbacks of aftermarket sending units, as well as how to make them work properly:

Fuel tank sending unit for 1966 Dart?

By chance when you installed the aftermarket sending unit did you ensure the pickup tube reaches all the way to the bottom of the tank?