I second Sean's diagnosis on the valve guides. My old Chevelle 396 did that.There was a big prob with that engine due to poor valve geometry. If you over revved them, it was said that the valve stems were "side loaded" and would take the valve guides out in no time. The cure was to either run roller rockers or ditch the 396 heads and install 427's. I sold mine before it got too bad but it was already going through oil,STP,spark plugs,etc. It did the same thing as you describe though, blue smoke out the tail pipes on decel. I would imagine bad valve seals would do the same....maybe even worse.