I would check the fuel gauge by grounding the wires for the gauge and the low fuel light, to ensure the gauge is working properly. Sounds like the gauge may not be working right, because you installed in tank pump assembly incorrectly and the float can not rise and drop accurately. Its a bit tricky to get the pump assembly in, but its critical to how its positioned to sit properly and provide a accurate fuel reading. While your doing this, I would also check the small step hose to be sure its not leaking and fits accurately. If its bad, you may get a sense by sound that the in tank pump is working but no fuel is being feed to the in-line pump, which can damage it.
'79 Alfa Romeo Spider