Slight vibration at 3400 RPM????

Could be engine harmonics. These engines are kind of thrown together with best guess in mind. The rods and pistons aren't exactly measured to within three decimal places like they are with an aftermarket set up after all. That and the stock balancer aren’t exactly precision built.

If it were a driveline vibration (drive shaft, u-joint, flywheel, etc), my guess is that it should be taking place at all RPM. You might want to crawl underneath and make sure none of the exhaust components are close enough to the floor pan, trans, or other areas of the vehicle to make contact.

You could try swapping out the balancer for a higher end one like ATI, or Innovators West....or even another Ford unit, but unless you know for certain, you might just be throwing unnecessary money at the problem.

Keep in mind...your Hurst shifter won't have the soft, rubber bushings that the stock unit did. It may have very well always been there, but not made itself prevailant until the addtions of the aftermarket shift. They've been known to trasfer vibration from the engine and/or driveline into the cabin.