I found a 7/16"-20 external to 5/16"-24 internal threaded inserts. They will probably cost more to ship than the cost of the insert.

http://www.globalindustrial.com/p/me...B&gclsrc=aw.ds

or

http://www.mscdirect.com/browse/tn/T...573+4288245784

Another option is to take a 7/16"-20 bolt from Home Depot, use a size "I" drill bit to drill down the center of the bolt and then tap it for 5/16"-24. Then you'd need to cut the bolt to the correct length to make it an insert. I'd probably use permanent loctite on it too.