Hi Dave,
First, obviously rule out the 1813 Birmingham proof if you want the gun to represent the 18th century. Then you need to decide if the gun is military or civilian. If civilian then the crown over V and crown over GP would be appropriate. That would signify a gun made within the jurisdiction of the London Gunmaker's guild (I believe within 10 miles of London). Two stampings of the crossed scepters (Gr-CS) would also be fine as indicating a private gun proofed at the Tower. That private tower proof also looks to me like Birmingham marks before 1813. I cannot tell the difference between those early Birmingham marks and the tower proofs but someone else may be able to clear that up. The civilian proof marks are always on the top of the barrel, left of center.
If the gun is British military then crossed scepters below stamp GR-A, which indicates government viewing and ownership and again crossed scepters on the tang, centered and near the junction with the barrel. Also stamp GR-A would be stamped on the lockplate below the pan with the arrow pointed toward the flintcock. The lockplate would also have a large engraved crown with 2 large script capital letters below indicating the reigning king or queen. All military barrel marks should be centered on the top of the barrel. Finally, storekeeper's mark GR-SKM would be stamped on the side of the butt, although I've seen some British arms without the storekeepers mark.
All proof marks, civilian and government, should be oriented such that they can be read from the breech with the barrel is held muzzle up.
dave