So, the other day I pulled the battery out of the Mirage and test fit the deep cycle that I had in the Metro. Turns out to be a pretty good fit. Its just a little bit longer, but identical in width and height.
However, the positive and negative terminals are on the wrong ends / sides... bleh. I'll have to run to the store and see what I can cobble together to make some extensions. There is no way the OEM battery cables will reach far enough on their own. Extra slack = none on today's cars it seems.