Twister, I'm guessing that you went up in elevation. In that case your engine would actually run richer. But if you were going down, 1000 feet wouldn't be enough to lean it out to a dangerous level. Probably wouldn't even notice much difference in perf. Everyone else is right though, iconel valves are pretty sweet. Expensive but they last. Just remember never to use bronze valve guides. They wear valves out very quickly unless you have valves with nitrided stems. But to be safe I'd run just enough water to keep the Sheriffs off your tail. I'd also double check everything else before I dropped 700 bucks for a set of valves. Do you have the heads off right now?