I've been watching this thread to see where it lead. I've been messing with these mirrors for years and also have a few sets here to measure. I have not worked on the Chevelle mirrors, just the F body and G body ones.
All of the early 2nd gen F bodys I've owned or worked on came stock with the ones which have the bracket that screws to the door. This type is easily recognized by the chrome screw that can be seen on the rear of the stem where it is held to the bracket. The sheet metal screws that hold the bracket to the door have a 2" bolt pattern. All the early 2nd gens I've worked on had this type. The design sucks. They use a couple of sheetmetal screws to hold the bracket in place and sandwich the plastic "gasket" which holds it in place.
The problems are that,
1. The rear end of the bracket bends easily. Someone leaning on the mirror once pretty much ruins it. Once bent the mirror wiggles. No amount of trying to push it back without taking the mirror off and straightening the bracket will cure it.
2. The sheetmetal screws that hold the bracket/gasket to the door get stripped by people who thought they were going to fix the loose mirror by removing the mirror and tightening whatever they found (the sheetmetal screws) not realizing the bracket is bent enough to make the mirror wobble but not enough to be noticable to the average inexperienced guy.
3. If the mirror wobbles for an extended period of time it makes a groove in the bottom of the gasket and tightening or replacing/rebending the bracket will still not cure the wobble enough to make the mirror feel solid.
It's been my experience that the ones held on by studs are MUCH more solidly held to the car. They have a wider bolt pattern (3 1/16 C to C on the G body one I'm measuring), and are secured with flanged locking nuts from the inside of the door thereby sandwiching the reinforced sheetmetal.
Tips I've learned.
1. The studs can be removed to aid in positioning the mirrors on a car which didn't have them stock. Just lock two nuts together on the stud and remove the stud.
2. There are gaskets of varying thicknesses and angles that were used on different cars. I've actually found that a better mirror alignment can be accomplished using thicker gaskets and sanding them off on the paint side to achieve the best mirror position on early 2nd gens, particularly on the passenger side.
3. If someone stripped the screws that hold the stem to the head (there's a cast aluminum piece inside the mirror itself is mounted to) self tapping screws can usually be used to attach the stem to the head.
4. 2ng gens windows tilt in quite a bit. When installing 2nd gen mirrors on vehicles with door windows more upright than a 2nd gen the mirrors with the longer stems aid in moving the mirrors out far enough to look correct. I have not installed bullet mirrors on a first gen but the 2nd gen stems may bring the mirror in too far toward the glass.
5. It's a personal thing but I don't like the writing on the passenger glass so when there's an option I use the ones without the writing.
6. Be sure to get the mirror positioned EXACTLY where you want it before tightening it up. I've straightened out crooked mirrors for people only to be disappointed that a mark can be seen in the paint afterward because the mirrors were installed on "soft" paint.
Shown below is a early 2nd gen mirror next to the yellow G body mirror.
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