Ultimate series I have no clue how any of it works (very different compared to ultimate choice and premium upgrades) but would love to know if any one does.

I think your best bet is either reserve the Shelby on the website (if available) or just make a full size pay later reservation and ask to upgrade at the gold counter when you arrive. I’ve had crazy luck at las but ymmv, the gate agent let me take a Maserati grecale on my full-size reservation(kept the same $34 a day rate)
but it was 2am and the agent was trying to get me out of his sight