How did you determine the bolts supplied are 5/16-24? M8-1.25 is very close to 5/16-24. It would be easy to get them mixed up. 24 thread is the common fine thread on a 5/16" bolt (source Mathinery's Handbook). However, since the M8-1.25 is commonly used on Jeep of all kinds, I'd substitute with the spec bolt: M8-1.25. Might try NAPA or a good Ace Hardware for the correct bolt. While you're there, ask for an all-metal lock nut. Jeep used a locking nut on the original. Use a washer with it since the flange nuts are difficult to locate. As with all lock-nuts, tighten snug, but no more (there are not torque specifications for lock nuts because of the way a lock nut works). Never reuse a locknut.
This wouldn't be the first time we've seen a manufacturer send 5/16 instead of M8...the 5/16 are cheaper and more readily available on the wholesale market.