Gryz...the batteries we are discussing ARE 12V batteries...just different sizes and cranking power.
JRP... that Lifeline looks pretty good to me. The MCA of 550 (MCA=CCA @ 0 degrees F) ...is pretty close to the Group 24 and if you have no problems with the 24 then I'd say it is worth a shot if you can fit it.
I would NOT worry about your charging capacity from the alternator at all. Starting will take 90 amps for 15 seconds or so...which means less than ONE amp hour to be replaced. Even a whole minute of starting is only 1.5 amp hours! About the same as running a cabin light for an hour!!
No longer posting. Reach me by PM!