Gas gauge woes..

Okay, I finally got time to work on my car.
Well, I did figure some things out.
Okay, My 1971 Valiant had a 1969 Dart tank in it, when I got it.
Okay, now that tank "dart" is a 18 gallon, and my "right tank for my car" is a 16 gallon tank. I put it in!!
Confused though, their the same dimensions, but one is more gallons? Anyhow..
I put my new sending unit in that I ordered on ebay. It is a "cheapo" one. That may have been my mistake right there.
The one that came out of my car, "remember it had a dart tank in it" had a sending unit in it, that was longer by about 2 inches. The float and pick up. But I figured it was longer to being in a dart tank?
I checked my ground today. Was fine!
I currently have exactly 7 gallons in the car...16 gallon tank.
I then pulled my filler neck out, and reached in with a long metal rod, and lifted up on the float, and my son yelled out that the gas gauge was going up. I lifted the float the whole way up, she went to full.
Okay, I then stuck the tank, and it measured 4 inches on the stick. The tank is about 9 1/2 inches. Tried to measure while it was in the car.
Right now it only shows about a titch above "e", when it should be right below a half a tank.
Do I just have a bad sender/float? As in the wrong one/Length? It does function, when I reach in the tank and move it manually.