Well....yeah. The one stock 440 I rebuilt was smooth as silk at idle, and smoother than most 4 cylinders....whatever.
The problem here is that we cannot possibly tell the vibration that you are feeling; the info is too limited to really say what it is: motor or trans mount? Driveline? Exhaust bumping around? Big motor? The advice to check with the machine shop is sound advice. They could have balanced it WITH the external balance flexplate or could have set it aside and internally balanced it; you just don't know at this point. The PO could have been told and not understood....
But do as you like..... you are spending $$ on a 50/50 shot of fixing the issue at this point, and asking the machine shop may help improve those odds. (BTW, this same discussion took place about 2 months ago here!)
Just one question: Does this flexplate have a 'bite' of material missing out of one edge? Or a weight on one side?
