This page should help you a lot:
Group 65 is very close in height but quite a bit wider AND deeper. Due to the width it won't fit exactly in the stock location unless you don't have an AC compressor or you cut the inner fender.
For massive upgrades most people find an older Optima (before their quality control took a dump) or those fancy Sears DieHard Platinum (same as Odyssy) batteries, or go the dual battery route. It really depends on your needs. Unless it's to power stuff while the engine is off don't forget to upgrade your alternator.
Regarding CCA and AGM spill-proof, unless you really expect to roll your XJ a standard lead-acid is fine for most people. In which case buyer should beware that higher CCA in the same group size isn't in the best interests of longevity. More power in the same format is achieved by increasing the density internally - that means lead plates are jammed closer together, and sometimes thinner. This reduces their ability to take shock and vibrations easily. If you look at aftermarket lead-acid batteries as a whole there's a strong correlation between "get the most CCA" and "dead cell after 1-3 years", but the low CCA OEM batteries somehow run fine for 13 years...
Food for thought. (If you aren't already stuffed with turkey lol)