Answer: While it is possible that the gas gauge itself has become stuck in the ‘Full” position, I think that it is the sending unit in the gas tank has become stuck in the full position, especially if you stored your car with a full tank of gas.
It’s relatively easy to check if the sending unit is the problem. Unplug the wire that’s attached to the sending unit, and with the ignition key in the “On” position the gauge should drop to zero. If it does, the problem lies with the sending unit or its connections. If the fuel gauge remains in the “Full” position, then this wire is “grounding” somewhere.
Since this happened while the car was in storage, I think that it is far more likely that the sending unit is simply stuck in the “Full” position. There’s a possibility that it will “unstuck” itself if you drain some of the gas from the tank and drive the car normally. A few bumps may jostle it loose. If not, you’ll have to remove the gas tank and “unstick” the sending unit or replace it.
The tachometer problem is difficult to diagnose other than to check for loose connections. If the problem lies with the unit itself, it can be professionally repaired, or replaced with reproduction units which are available. Other than removal of the dashboard faceplate, the dashboard does not need to be removed. Calibrating is done by measuring the RPMs of the engine with an external tachometer, and then turning a small adjusting screw on the back of your in-dash tach until they match. It is not unusual to be unable to achieve a perfect match across the entire RPM range.