It may be slightly different, but I may be able to help you. Just yesterday I was fixing my off-track window in my 72 Newport.
In mine, the window (drivers side) is mounted to a horizontal bar on the bottom, and a vertical track on the right. On the track (from back to front)
slider -> nylon washer -> window -> rubber washer -> wide, flat bolt. A screw goes through the back of the slider, all the way through the bolt. when I tried to loosen this, it just spun too. What I did was push as much pressure as I could on the nylon washer with my hand, to hold the screw still. Then I carefully spun the bolt off. It was difficult, and I was cutting my hands the whole way through. I don't know if the design on a 78 would be that different. I'm sure that it is similar.
If you can post a picture of where the window is mounted, I may be able to help you.