6. BATTERY: Only a lead acid or gel cell battery ( 25lbs. min.) may be used. The approximate dimensions are 71/8” X 8 X 61/2”. A top post or side post battery may be used. Motorcycle batteries may not be used. The battery must remain in its stock location and securely mounted. The original stock battery bracket may not be altered in any way. A battery box, terminal coverings or rubber padding around rear end is also highly recommended. A battery shut-off switch is optional.
Battery box offset is is listed as 5 1/2" from the frame rail in the Appendix. No mention is made as to the physical size of the battery box dimensions.
Perhaps the extra room in the dimensions is for a battery box or padding around the battery. Padding could certainly cut down on some vibration of the battery.
Specific questions as to the rules should be made directly to INEX.
Steve