Hmmmm. A head scratcher for sure. Only two things can be happening. Either the flex plate is wrong or the starter is wrong. If the engine and transmission were separated [as in engine swap] then it is possible the flex plate is not correct or mounted wrong. Then of course starters are easily swapped so who knows what that one actually belongs to. This may actually be why the car was parked for so long? Someone did the engine swap, and the starter kept getting hung up? What I know about Pontiac flywheels is that they are drilled so they can only go on one way- the right way. I have swapped endless flywheels between engines and never a problem, even manual flywheels. I would think it is safe to assume the flywheel is OK. But starters are a different story. Olds and Pontiac interchange. All others mount to the passenger side. All Pontiac V-8 starters interchange except for the 301 which bolts up but is much smaller. The only thing I can think of is getting another correct Pontiac starter and seeing if it works better. If not then there is a different problem.