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.
A unbalanced engine with a stock oiling system isn't going to stay together very long with those kind of rpm's. Even with quality machine work and forged parts with that kind of rpm's the engine isn't going to last long.
Did you read that somewhere or have you broke a bunch of 340's? My 340 was balanced, Ross flat tops, Eagle SIR (cheap) rods and ported Edelbrock heads. It got turned 6700 with the stock oiling system. It did have a HV pump and a Milodon pan. After five years of street/strip use the bearings looked fine.
