I have a 1970 Cutlass 350 & 4-bbl. Before heading to the gas station, I added an ounce of lead substitute, then drove down the block and filled up full with gas. Just afterward, the fuel gauge was reading way past Full; the needle was on the H for the Hot light. It has never done this before. It has backed off only slightly, it is still a little past the Full mark after 120 miles of driving. Is it possible the lead substitute wrecked something in the sending unit, or did I loose a ground somewhere??

 

Break in the wire causes it to peg out at high. Don't know about the lead, but doubt it.

 

X2.
Could also be just a bad sender.
If the plug to your sender is in the trunk, unplug it, and jumper it to a good ground. When you turn on your ignition and it goes to FULL then you have a bad sender. Or possibly the ground wire came disconnected.