The 68 Chassis Assembly Manual is no help in this case as the shield is installed as a sub-assembly and individual part are not shown, so to use Ford Car Part is about the only option left (yeah, I know, find the same year car, with the same equipment, from the same plant, etc.) to determine what to use. The bolt you reference, 381985-S is (as documented in AMK Guide to Ford Fasteners) as a 5/16-18X1/2 hex head with a recessed head, 3/8 drive, split lock washer, and a nylon locking patch. AMK carries that bolt under their stock number B-10911 (2010 cat, pg 190).
Jim
PS - get a copy of AMK's Guide to Ford Fasteners.