Some resolution:
From the KMS website,
We place two of them side by side into the 427 roadster, behind the passenger seat. We strongly recommend using SEALED batteries, since sulpheric acid dripping about in hard manouvering will cause problems.
We get ours from Batteries Plus, and you should be able to get a pair pretty much anywhere. It's 12 Volt (2 are run in parallel) 35 Ampere Hours.
The largest dimensions:
8 5/8 inches wide
6 inches high
5 1/8 inches deep
At the base:
5 inches by
7 1/2 inches
which fits our 15 1/2 inch by 5 inch battery tray.