We fired up my 340 after it rebuild tonight. Only problem I have is a vibration that you can feel shaking the car at idle.The motor itself is running smooth. The only thing we can think of is there is a problem with the flexplate/torque converter hookup. Is there some sort of markings to get correct lineup and balance? the book I had made no mention of marking things or alginment markings upon installation.

What year 340? Cast crank or forged?

It's a 1973 340 motor

Do you have the weighted converter or neutral balance converter with the flexplate for a cast crank 340 and a cast crank only 340 damper?
There is only one way that the flexpate can be mounted either to the engine or to the converter.

The motor is a 73 340 motor. It did not viberate before we took motor out for rebuild.The harmonic balancer,crank, flex plate and torque convertor are all the same as it was before rebuild. The only thing I have found now after looking at factory service manual was the alignmnet of flexplate to torque convertor. there is a small hole in plate which has to align with a > on torque convertor.And the drain plug is 180 degrees on oppisite side. I didn't realize this when I bolted everthing back up. Could this
be my problem? There was no mention of any special way the flexplate aligns with crank with markings when bolting it up to the crank.

As I said before there is only one way that the flexplate will fit the converter, and only one way that the flexplate will mate up to the bolt holes in teh crankshaft. Vertually impossible to mount wrong unless you redrill the bolt holes as one is offset.

Did the crank get re-ground? If so, are you sure you got the same crank back?

My engine builder also did the machine work. The crank didn't have to be reground. He only works on one engine at a time. He is also very picking and organized when it comes to rebuilding motors.