Won't Shift Into Gear While Running

More than likely Rusty's on the money. Do you know how to set the free-play? Which sets the plate departure, which is what you need more of.
While it is possible to put the disc in backwards, it is unlikely, since they are always clearly marked "flywheel side".
It's also possible that ,having sat so long, the disc is stuck to one of the the machined surfaces. This doesn't usually happen to cars, but I have seen it a lot on motorbikes.Using Rusty's trick will almost instantly prove if it is or is not stuck. If the engine is able to run at all with the rear brakes locked, and the pedal on the floor, then there is at least,some slippage .
The Zbar plays a critical role in getting enough departure.