The Jeti 600 will work. Its good for 60A!

I'm concerned about the GTX OB. It is very limited. I looked in to this a while ago and found threads saying people had an extremely hard time finding parts. Many people did not like the 4mm prop limitation for a high powered outboard. You will make somewhere over 700 watts on 20 cells.

If you went with a 3.5cc lower, you could still use an 800 motor. It will be far more reliable and more efficient. You can also run metal props.

Ryan