I have a '90 Camaro with a 350 and a bad vibration problem, even idling in park. Long story, so I will start at the beginning.
I also have an '85 square body C10 with a 350 out of a '94 Suburban.
I put a crate long block 350 in the Camaro, vibration was horrible. After doing everything everybody said, the vibration was still there.
In a fit of desperation, I swapped the new 350 into the truck and the '94 Suburban 350 into the Camaro.
I expected the car to be smooth and the truck to vibrate as soon as I started it. I was wrong.
The car still vibrated.
Recently I had it up on a lift and had to start the car. All 4 wheels were just hanging there. The car had zero vibration.
My question is this, could the K-Member, lower control arms, struts, springs, bushings, or gremlins cause the vibration with the car idling in park with the chassis under load?

I haven't found any posts quite like this after much searching.
Any and all help is appreciated.

 

torque converter out of balance or badly worn torque converter bushing is all I can think of.Is vibration at engine RPM or more of a "grumbling"vibration like engine/tranny in contact with body ?

 

New torque converter, fresh rebuild on the transmission. The entire car shakes.

 

Does the flywheel have a counter weight welded to it?

 

I think folks are suspecting an internal/external balance issue, yes? I don't recall the details but I kind of recall there being balance issues with 1-piece, 2-piece rear main seal cranks, flywheel/flexplates and harmonic balancers and *****-nilly mixing and matching these parts.