A couple of questions come to mind...1) has the engine been balanced as an assembly? 2) what size are the bolts you are using (Hemi converters use oversized bolts probably for that very reason). Of course, you couldn't be using the large holes in the flex plate with small bolts...could you?

Years ago, I fixed a bad rattle in a GM car that was traced to loose converter bolts. The young lady was very grateful