Im planning on gutting my airbox on my 600ho and running 400 mains .Could someone explain how the pressure change in the airbox would require a change in jet size, i realize the effect that leaning out the main jet has on the fuel mixture but isnt there more to it when you figure in running a gutted box to a box with the shelf still in place.

This probably wont be the most popular response, but this is what I have found over the years.
Most carb polaris sleds have the carb venting nipples below the shelf. Which in turn creates a negative pressure to the float bowls via the vent tubes due to the intake pulses of the motor. Now you take the shelf out and open the flow to the motor. The vent tubes are then subjected to positive pressure which inturns will transfer into the vent lines causing an enrichment to the main jet.
Due to this (on a polaris) it is rarley needed to jet up.(disclaimer- Even though piston wash and plug color should aways be checked to correlate a safe condition)
Now an arctic cat or any other sled that are not vented to the airbox will need to have the jetting checked closely as it will definitely create a leaner condition.

Just my .02
Mark

^^^Yup