Godlogs57 is right. If a 3/8” lag bolt in a stud doesn’t work , you may have operator error or worse problems, just saying. 2x6 turned flatwise on 16” centers and lag bolts make it pretty simple and gives a lot of options . 3/4 plywood is unnecessary expense. 1/2 “ osb works fine if it makes you feel better. A big bull elk is heavy and awkward to hang. 3/8 lag bolt will hang anything.
