There is only one reason for the stud to be larger than 5/16. The rearmost upper stud seems to be the most prone to breakage. So that stud was broken sometime in the past, and someone just did a drill and retap to a larger size. It is really not a good idea to use a larger stud. The exhaust manifold is designed to "float". As it heats up, it expands and gets longer. If an oversize stud is used there might not be enough clearance between the stud and the hole in the manifold, which could possibly snap the stud, or crack the manifold. The best repair is to remove the broken stud, and use some type of thread repair method (helicoil, nutsert, etc), to bring it back to 5/16-18 NC. Just remember the studs do go into water, so make sure to use a sealant.