Internal balanced..what damper do I need

The 408 is based off a 360. I have an internally balanced 408, a flex plate for internal balance, a 10" 2800 stall converter in a 200R4 transmission and a Fluidampr balancer. I've been fighting a vibration for years. I've tried driveshaft balance as well. Mine is rhythmic and comes in around 60 and higher. Sometimes it goes away when I let off the gas, sometimes it doesn't. I wish I had an answer for you, brother, but I am subscribed and will be watching to see if you fix the issue. This has been a three year problem for me.