Thanks Steve. Glad you're feeling well enough to respond!
The OP is working on a 69 Roadrunner and I couldn't remember if the gasket unzips like an A body? I restored a Superbird back in the mid 80's and the guy at the local PPG glass place came and installed my new windshield for free, just to be able to say he put one in a Superbird! He used a small rope inside the car after setting it in the bottom of the opening. Took him about 30 seconds to rope it in and it never leaked a drop!
I've done 2 Dusters, exactly like you described above.