Please help me diagnose a vibration in my Dart.

'73 Dart Sport 340.

I reinstalled the engine and transmission about a month ago and now there is some kind of vibration. Prior to the removal, the car rode really smooth.
The vibration is felt in the steering wheel and is really obvious below 35 mph. It is not steady, it pulsates. I don’t feel it in the seat of my pants.
If I stand on the gas in first gear and run it to 40 mph, then let off the gas, the steering wheel really vibrates.
I didn’t do anything with the wheels and tires; they stayed on the car. I rebuilt the entire front end two years and had the front end aligned. The tires look to be wearing evenly.
Am I looking at a tire balance issue or could it be something related to the drive train?

Thanks.