[chuckbrandt]

I think it depends on if the 390 was balanced and if to acheive the balance they changed the flexplate or not.

If the 390 had been balanced and modifying the crank counterweights was not enough to get it in balance they may have modified the flex plate. In that case if you bolt on a zero balanced flywheel you could have a problem.

But if you are getting a running stock or stock rebuilt motor I think you will be fine and the above scenario is unlikely. In fact I did exactly that in my pickup. I bought a running '65 352 4V out of a galaxie and put it in my '66 F-100. I took off the flex plate, added a pilot bearing and flywheel and put it in. I had no problems at all with balance.

What do they call it, "detroit" balance?

That's what I'm assuming by my experience, please someone correct me if I'm wrong.

Chuck