Sitting in a shop for 6 months is enough to cause corroded battery terminal, starter terminal contacts. Remove and Clean all the connections.
Remove the battery and charge it. It may have sulfated from sitting.
Get a heat shield wrap for the starter motor.
Replace the GM solenoid. They don't last forever, especially with headers.
Install a ignition power interrupt switch to allow cranking the motor over with no spark. Get it spinning then hit the spark.
Ignition timing, mechanical advance weights stuck advanced.