Check out that link and the 9A34M. That should be the same size or very close to the Optima 34M. It has the same 20 AH rating and same reserve capacity but I bet can be had for far less than the Optima's.
John, I just measured a couple of the batteries and you would be good with group 34, 24, 27 or 31. They are all under 10" to the top of the terminals.
I had Exide's in my most recent boat when I bought it last year. The previous owner had just put them in a month or 2 before. I tested them and they were overrated even after putting a good long charge with an industrial charger on it. I immediately replaced them.
John, if you are in the SODO area of Seattle let me know. You are more than welcome to stop by and take a look at some stuff and take your own measurements.
(Message edited by polarbill on March 05, 2009)