there should be an adjustment on the back of the guage.
basically what you are looking at with your fuel level isnt how much fuel is in the tank, its how much resistance the gauage is reading between the gauge and earth. most senders use the same resistance, so most guages work.
i dunno. i have never heard of of an aftermarket fuel pickup that goes in a stock tank.
you can test the guage by touching the wire onto the body of the car (away from fuel pls, it may spark). im pretty sure that grounding it will show that its empty, but it may be the other way round. remember it needs to have ignition 'on'
you test the sender by putting a multimeter on the body of the sender unit and the plug, when you move the float up and down the resistance should change.