Engine Vibration 3k RPM

Hi everyone, I'm chasing an engine vibration (or at least that's what I think).

70 Dart
68 340
A500 OD transmission
Neutral converter
Neutral balancer

The vibration can be felt in Neutral when revving the engine to 3-3500k. It's not as pronounced as when it's wheels on the ground, at speed though.

The vibration goes away when the OD kicks on, which drops it to about 2200 RPM.

I thought it was driveline or pinion angle related, but I took it back to the guy who built the driveline and rear end and after we went for a drive, he doesn't believe it's driveline related. He even had another guy come by and get his opinion.

The trouble is, this is a new build (8+ years in the making) so I don't have anything to go off of.

Here are things,.I was planning on checking:
1. I was thinking maybe a misfire (not sure how to check)
2. It's got Fuel injection with timing control (maybe rotor phasing is off)

I wanted to remove driveline to be absolutely sure it's not rear end or wheel related, but not sure the best way to plug the trans. Will these work? https://www.oreillyauto.com/detail/...le-transmission-transaxle-plug-set/lis0/23400

Thanks for any suggestions.
Lets do the easy things first.
Check the torque converter bolts make sure they a tight and loctite has been applied.
Remove all the drive belts from the front of the engine, water pump, alternator, etc with the car in park check for your vibration you may have a fan or pump that's going south. Make dam sure you do not overheat the engine doing this, start with a cold engine.
Post some close up pictures of the spark plugs so we can see the timing mark on the ground strap.