You will have issues down the - those batteries do not have that amp hour capacity that an industrial batteries has - those batteries will have a shortened life real soon. When the motors are under load - lift, tilt, side shift steering and drive function they will draw down the amps of the battery quickly and the battery voltage will drop below 36 v (undervoltage condition) and electrical & electronic components will over heat, brushes wear, etc.
Realtive to safety & potential liabilities & OSHA - no one on here will approve what you are doing -you are on your own. The days of back yard engineering are history - gotta' follow what the manufactuerer says and say not to do.
Good luck