So here is what I was up to. Doing some research, the 911 guys do this battery upgrade and confirmed an owner with e30 m3 street version car did it as well. There is an Odyssey battery, the PC925, that is much smaller, still AGM, and about ~12lbs lighter, and also cheaper than the 96 I bought. Because I was within my 30 days, Autozone happily exchanged it and credit me the difference.
I also found a company that makes a kit to mount it. Company is call Rennline, same as the one with the phone mount I installed. I purchased the universal kit and bolted it with 2 bolts to the existing e30 plastic battery tray. I did get measurements in having conversations with them. Great customer service.
Here are the old 96R batter compared to the new 925. I did have to by the SAE terminal posts which is nice as it has a bolt that screws in on top. Used it for the trickle battery charger connectors.

The posts (from Odyssey):

The battery installed with the Rennline mounting kit:
