I am running the largest group size Optima that I know of, model D31A yellow-tops. See my reader's rigs gallery for pictures. Unlike the Odyssey group 31 batteries or the standard group 31 batts I considered, the Optima's require very minimal alterations to the stock setup.
The passenger side battery slips right into the tray with no modifications and sits perfectly flat. The hold-down bolts must be lengthened or replaced (I lengthened them by cutting them in half and adding a length of bar, then painting).
The driver's side required that one of the tiny gussets at the bottom of the factory tray be removed (I used my dremel) in order for the battery to sit flush against the temperature sensor. The bolts also had to be lengthened.
Other than that they fit great and, IMHO, look great too. They have 900CCA and 155 mins RC which exceed stock requirements.
As many have mentioned, you can get more CCAs and RC mins with other batteries. My choice was dictated by a battery that will not eat my fenders and that does not require extensive battery tray modification.
Also, to offer an answer to the original question in this post, these guys sell Optima trays as well:
http://www.odysseybatteries.com/accessories/optima.htm
The TDR is an amazing place!! I researched my options here for a long time before making a decision and I humbly submit the above information as my personal experience on my truck. Many others with more knowledge and experience have made different choices.
Thanks,
John