Fuel gauge

[Smoke 1969]

im looking for some help with my fuel gauge issue, I bought a new fuel tank from Boyd's and a brand new complete dash from truck shop oc , my problem is that the fuel gauge reads full till the tank is half, then it starts to go down, I contacted Boyd's thinking it would be the sending unit and they were excellent giving me info how to check the sending unit, it to read 0 - 90 ohm for this 69 truck and it's right on, just wondering if any one else had this problem and how they fixed it, here is a couple of pics of my truck and how it's coming, just waiting on the box and I'm ready to go, thanks in advance everyone