This is what you quoted, so where did the unbalanced part come from? Back in the day I knew a bunch of guys that regularly turned their stock 340's 6k with no ill effects. Yes eventually you can break anything, but if your engine is healthy it'll be fine at 6K.
I had the rotating assembly rebalanced as I used a lighter weight pistons than the factory bucket's. I was amazed how far off the factory rods were, and these were the original set of connecting rods not a mixed set from different engines.
