You can screw a piece of wood to the inside of each box to bring the internal volume down to 1.1. There are a bunch of cubic foot calculators on the internet that you can use to determine the proper size of the wood. The subs would have more punch with the full 500 watts, but my guess is you'll definitely know they're there with 400.
Is the area you're installing the boxes relatively dry? In my experience, the MDF that's used in most aftermarket boxes doesn't do well with a bunch of moisture.