Another Heater Box Question

[dfsmopars]

I am goint to replace the heater core on my '72 A/C Charger. There is one of the screws across the top of the box that is directly behind the dash vent duct. I can feel it but there is no way I can get a nut driver or wrench to it. I have already removed 15 screws and I am assuming this is the last one. How do I get it out and are there any more screws than these 16? According to billfury's post there is a gasket that has to be replaced so I am assuming this gasket will make it difficult to separate the two units. Thanks for any help.