The delay is only for one year, the original date was 2019MY and MAYBE now it will be 2020MY. They wanted to leave the "halo spot" to the GT350 for a while.
I'm not sure about that. Both main mustang forums initially posted forum subheadings 2018MY GT500. Then it was moved to 2019, but I'd probably guess they are going to change that again here shortly, and if it isn't, there may just be a 2019 MY GT500, but a small run. Possibly a few hundred made between march and 2020 MY change over? Who knows but you can sure bet that would be a tough one to get, in fact even the first half of 2020MY will be tough to get one.