Looking at the 3513 rubber insulator a bit more closely, I compared the few that I have to what is currently available at a large national supplier. The first pic shows their 67-68 version, along with the 69-70 for contrast.
As these things tend to go, so there should be no surprise, the replacement part does not match the factory ones that I have. Those are the subsequent pictures.
Starting with the easy stuff, I'll call the firewall surface the X-Y surface. The original parts, as best as I can figure, follow the flat plate 3B633 outer shape in all dimensions, including the radii. Moving inward, the four mounting holes line up, but the diameter of the holes in the rubber are about 1/2" whereas the steel retainers are at 5/8". Moving toward the center, the hole in the rubber is much smaller, providing a seal against the column, whereas the steel provides quite a gap. I would put the rubber at about 1-7/8" and the 3B633 plate at just under 3.5". Quite a difference indeed.
I need to note here that the rubber seals are about 1/16" thick, but they are not "flat". There is a "Z" dimension that allows for a proper seal against the column. I would peg that "Z" dimension at between 1/4" and 3/8".
If you look at the catalog (first) pic, you will see that the 67-68 part is flat as a pancake. I wonder by the way if they are selling like pancakes, but I'll get back on task. The 69-70 has a wild (in comparison, no offense intended) Z dimension compared to the 67-68 part. Looks more like a manual shift boot. Anyway, the factory truth for 67-68 is in between those two.
Part of the problem that I see is that when I pull these out of original cars, they are knarly, dude. Probably installed in a hurry, then exposed to the elements, time, etc. This is why some of the dimensions I noted above are "about" this or that.
Note that the two piece retainer has a slightly different profile. But the 3513 rubber parts all look pretty much the same. I can tell which one was under the two piece retainer, since some of the rubber seal got caught in the gap. But when stretching it out, it looks to be the same as used under the one piece retainer