Mark - I'm 99% certain those pan bolts are 5/16-18. But have someone else confirm before you go buying a helicoil, tap and inserter tool!
Another (probably cheaper) option: Drill and retap the stripped hole the next screw size larger (3/8-16). Then use a 3/8-16 bolt (or a 3/8-16 socket head cap screw if a 3/8 bolt head is too big) in that hole. Of course you would need to drill out the mating hole in the pan flange to fit the larger pan bolt.