I agree with Dave, ..That's really high for the reverse flow plate, and wasting a lot of room in the cooking chamber.
When you say firebox is at 90% volume, are you saying the firebox is sized at 90% of the size of the cooking chamber, or is it 90% the recommended size for that cooking chamber? In your research, have you checked out the pit calculator? I recommend shooting for a firebox 120% according to the pit calculator. I generally install the reverse flow plate approx 1/3 the height of the cooking chamber, checking the pit calculator, and install the bottom cooking rack at dead center. Those specs, along with some other important details, always produces a well breathing , even temp pit. Almost always, when I see an attempt at a reverse flow that does not follow those guidelines, they have problems.
If your stuck having to mount the firebox really high, and cant engineer around that fact, I'd think about a different design cooker. I have been thinking about a cooker that has the firebox to cooking chamber above the cooking rack, as with the jambo pits, and plan on experimenting with that soon. But really cant give any advise because I have not built or operated that style cooker yet. Maybe someone else has and can give some performance reviews.
Here is a typical 500 gallon cooker configuration for a reverse flow, as you can see , it has plenty of ground clearance with out the cooking chamber being too high. If you are struggling with placement issues for a difficult installation, inform us of the installation requirements and maybe we can help you figure out some options there.