I went to a local high-quality fixture store just to look today. I know it's not theoretically possible to get the Drake into my small space, but I just want to try it out.
They had boxes with Drakes just sitting there and they also had Cadet 3 bowls with the 12" tank, but no 10" tanks. I talked to them a little about it, we talked about my small space, I convinced them that I'm pretty careful and they sold me a Drake with the deal that they'll look it over carefully if I have to bring it back and get a Cadet 3 with the 10" tank. If it doesn't look brand new they'll charge me a 25% restocking, which I think is fair. I'm gonna dremel off those extra bolts, clean the heck out of my dirty flange and floor, lay down some clean rags, and try setting the Drake bowl on the bolts. If it fits I'll try setting the Drake tank on the bowl. If it almost sorta fits I'll think about it for a while. Maybe I'll take those bolts out of that old cast iron flange, cut notches 1/4in in front of where they are now and put them in again and see what the situation is now. Or drill into the concrete floor and screw some toilet bolts right into to the floor instead of the flange with the assumption that the outside of the Drake funnel is a bunch smaller than the inside of my 4" cast iron drain so I can sort of set it in the front of the flange rather than right in the middle.
Anyway, now that I have it home I can make much more detailed measurements than the spec drawings. I'm an experimentalist by training, ask lots of questions, do lots of reading, try things out, if they don't work try something else. It's a luxury that real plumbers don't have cause they're getting paid to do a job and don't have time to screw around. They'd put in a 10" rough-in toilet and move on to the next job. No one knows what I might do. I just might have the wall behind the toilet out in a few hours so I can screw smaller studs into the foundation wall to make a niche for the toilet. Or I might have unscrewed the Guinevere from upstairs (which has a 10" uni-fit on an 11" rough-in), lugged the beast downstairs, Tapconned it into the floor in the basement, and put the Drake upstairs where I'm more confident it will fit. Dunno.
Or maybe I'll see the thing in there and decide that I should just order the Cadet 3 with the 10" tank and have a much easier time getting a second toilet in the place.