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.
No longer posting. Reach me by PM!