Hey Tim,
Your mower is starting to look like early versions of the Electro-Metro, when it had lots of wood supports in it!
The Group 31 Gel batteries are a little wide too fit in cross-wise in the Elec-Trak. Since your mower doesn't really have a front end on it, that might not be an issue for you.
I do assume you will do something more classy than 2x4s when you are all done!