This may not be the problem--but in the off chance it is---There is a raised baffle wall at the bottom of the tank which is circular in shape and offset from the opening at the top of the tank for the sender. If the tank unit had been replaced previously with a near full tank it would be possible to install it with the float arm facing the wrong way leaving the arm to catch the "wall" when the level gets lower. This would be easy to check by observing the direction of the connecting tubes at the top of the sender. They should basically be pointing towards the front of the car.
And to answer your last question - it is not possible to check the resistance range without removing the sender and swinging the arm through its full motion. -- Dave