As long as the "pancake" doesn't drop below the spring shackles you will be all right. As for turning a bumper into an air tank, it is doable, but the bumper needs to be built with this thought in mind. All welded parts need to be open to allow air to flow from one to the other and the welds have to be perfectly sealed, no porosity. You also need enough cubic inches available to provide a reasonable amount of air. I have seen one buggy that was built on a tubular chassis and the entire chassis was one big air tank. It took a lot of planning and careful welding though. I also have a friend who builds tube bumpers for 4WD trucks that are designed for air storage. Make sure whatever you do that you have a covered spot for the air fitting. It will become clogged with dirt and mud if you don't. My friend that builds tube bumpers builds a box with a latched door for the air fitting.