If I want to set up stock, off-the-showroom-floor ride height on a 63 signet,
how high should the front and back of the body be, and how is it measured?

The way the book gives to measure the front height only measures the angle of the LCA, not the actual distance between the ground and the body.
"Measuring Rear Spring Height" on 17-2 deals only with the side-to-side difference in height.

Is there a body servicing book that gives these numbers and measuring techniques?

just a quick thought on this, if you replace the rear leafs with new ones that are correct for that year and car then that should be as close as its gonna get for the rear height, the i would just adjust the front to match so that it was level, seems like the easiest way to get it damn near perfect

Just looking for something to compare my car to.
If there was a restored 1963 Valiant in a showroom somewhere, I'd take measurements from it as a reference.
The springs are bought and installed and the torsion bars have been adjusted.
Now I'm trying to figure out if it's too high or just right.
Should the bottom edge of the body be parallel to the ground?