The biggest cause of violent gear engagement is a higher than normal idle speed. Try slowing the idle down to 550-600 RPM and see if the problem diminishes. Some of those transmissions had plastic thrust washers between the rotating assemblies, so the plastic you're seeing might be the remains of a thrust washer that's been damaged by the slam-bang engagement.
Also check the universal joints, carrier bearing (if it's got a 2-piece driveshaft) and the rear end for excessive clearance anywhere. If the driveshaft gets a good spin before all the clearance is taken up, the jolt can mimic hard shifting.
Jerry
PS: If the black plastic debris is in the form of thin strips, you've got a clutch piston seal damaged, which could also account for the hard engagement. A leak from the case vent is caused by overfilling the oil pan before you start the engine and put some of the new fluid into the torque converter. That, at least, should stop after some running time for the fluid level to stabilize. If you've got bad clutch piston seals, it's overhaul time, as driving it that way will cause the clutches to slip, and burn out.
Good luck!
Jerry
