[FirstOwner69]

The original DG4s on my '69 did not use "bolts". They (pic attached) were what I'd refer to as large hex head sheet metal screws. the threaded portion is 3/4" long. The head takes a 3/8" socket. The threaded portion diameter is slightly less than1/4". So, I assume a 3/16" hole may have been used. The the mirrors bolted directly to the paint... no washer of any kind. Unless you are striving for complete originality, you may want to consider using the method GM used on vans around the late '70s for mounting the large outside mirrors (the second pic).