Here's what I think I'm going with. It is a sealed gel cell battery with correct dimensions for the tray (no need to modify as in the case with the two 6volt optimas). They are for the Mazda Miata and have 475 CCA each, so wired in parallel they'll be 950.
Voltage 12
Amp Hour 31
CCA* 475
Length 7 1/2"
Width 5"
Height 7"
Weight 25lbs
MSRP $84.95
http://www.westcobattery.com/miata.html