Fuel Gauge problems

In my 68 chevelle, the fuel gauge does not seem to work. I've searched this board and tried every suggestion. If I disconnect the tan wire it will go to the full mark. If I uplug the fischer plug above the fuse box and ground the tan wire on the dash side, it only drops to about 3/4 tank. Does this mean the gauge is bad?
Any suggestions on where to get a new gauge? Can I use an after market gauge and put it in one of the multi gauge clusters?

Thanks

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1968 Chevelle SS396