Harmonic Vibration 3500-4000 RPM

If it vibrates around 3500 in each gear, it sounds like it is part of the engine. The drivetrain rpm would be different at 3500 going through the gears.

Did the cop car have a 318 or 360? If unknown, check converter for welded on weights. 360 was externally balanced.

The poly engine mounts could make any vibrations more noticeable, even if something isn't actually bad. But do check the other suggestions as well.