I'm far from an expert on ducting, but I'm pretty sure a 5.75" high plenum will not work. Air velocities in the 5.75" area will be high enough to cause severe noise issues and the backpressure could be too much for the blower to handle. I'd suggest either:
1. Reframing the floor joists to create an opening for a taller plenum.
2. Extending the plenum parallel to the floor joists--inside a joist bay--and connect your 7" ducts to the underside of the extended plenum.
3. Converting your AHU to horizontal mount or replacing it with a horizontally mounted AHU.
I don't know which of these options will work best. Consulting an engineer with experience in CFD for ducting might be in order.