I've always assumed my VO2Max power was roughly equivalent to the highest power I could put out for 5 min. In your case I suspect that would be considerably below 343W.

Whether you actually reach your VO2Max oxygen uptake during a given interval is a function of power and time. If you go just a little over your threshold power you will eventually reach VO2Max but it will take longer than a higher intensity interval.

I would stick with steady state intervals for the most part as they are more repeatable. You can use an all-out 3min test to estimate your threshold power and anaerobic work capacity (AWC). In this test you just go as hard as you can from the beginning and try and maintain power. Your power will naturally drop and level out around your threshold power in the last 30 seconds.