The easy way would be to remove your heater and driver side air vent. Or you could pull both fenders, the hood, pop out the windshield, drill out three hundred spot welds, and then you could clean it from the top.
This I see is the only way. While your at it (cleaning) you may be replacing the vent system. If the hats are good, I would clean and remove as much rust as possible and protecting the hats with POR-15 or similar