Interstate told me they list a Group 35 for the Mirage as well. It fits, but the original J-bolts may not be long enough since the Group 35 is taller than the Hitachi battery that comes with the car. AFAIK, the only difference between a 35 and an 85 is height. I know some other forumers who have used Group 35s have had to replace them. As you can see in the pic of the OEM battery above, there isn't much extra length on the J-bolts. Not a big deal, but worth noting.
The Group 85 just drops in.
Pretty much any replacement battery will have much more reserve capacity than the somewhat puny OEM battery that comes with the car. It has 356 CCA, this Interstate has 625 CCA.