Well, I dont know how different the later 67-& up A-bodies are, but I kinda made a mount to fit in the stock battery tray on my '68 fastback.
I first got a huge piece of 1/2" thick aluminum, then cut it to fit inside of the stock battery tray. The 3 larger countersunk holes are where the factory battery tray mounts, and I wanted to fasten the aluminum plate to those points, so I just used longer allen head bolts.
Here it is mounted in the tray.
Then I found this batter tray I thought was neat, for the Odyssey batteries.
http://www.qualitypowerauto.com/catalog.php?item=548
And mounted it to the aluminum piece.
All together.
The battery I used is a Die Hard Platinum, available at Sears. They are the same exact as the $300 dollar Advanced Glass Mat Odyssey battery. Made in the same plant.
Anyways, thats what I did. Thought it was better than the hoakey *** factory piece that hooks into the inner fender....