From what I’ve read here on the forum, the people who have fit in “golf cart” 6 volt batteries have done so by putting a spacer under the top step to gain height clearance for the batteries. They report success that way, and no significant difficulties.
As I remember, they used a 1/2 or 3/4 inch wooden frame around all four step edges.
2012 Solera "S"
Calif SF Bay Area