[camaraderie]

The starter is 1000watts and at 12V that is 83amps + some loss. Let's call it 90 amps of current. A 240MCA battery can sustain 240amps for 30 seconds before becoming TOTALLY dead. I think you will be cutting it very close with that battery especially since 3GM's have a rep for being difficult to start. Does yours take a while to get it to turn over?
If you want to try this....Sears has a Gold diehard garden tractor U1 size with 340CCA's (more stringent than MCA's) that may give you a bit more margin for error.

 

[camaraderie]

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!!

 

[camaraderie]

JRP...the Odyssey 1200's are NOT much more than the Lifelines.
Odyssey Batteries and price list

 

[camaraderie]

JRP...Funny...I had looked at the Odysseys compared to wet cells in price and said "holy xxxx" those are expensive" ...compared to the Lifelines though they aren't bad. If they do what they say...that should put a lot of price pressure on Lifeline, Rolls, Optima etc. etc.
I don't think there is much advantage for a starter...but house batteries with that price differential would be quite attractive given the many benefits over standard AGM/Gels that are touted.