yes, the GNX had 16" wheels because they thought they were going to be using the 12" brakes from the C4 Vette on it..
i would imagine that they just used the same spare tire and jack as every other Regal. i don't think i've ever heard anyone talking about a special spare tire for those cars- but i can say from experience that the stock spare tire in my 84 T Type does fit over the 12" C4 Vette brakes that they almost put on the GNX but decided not to for budgetary reasons... i figured that out back when i first got my T Type in '04 and i wanted to see if the spare tire (which i don't think had ever been out of the car at that point) would fit over the 12" C4 front brakes that i had on my Nova..