I'd never seen this tool before you posted here:
https://67-72chevytrucks.com/vboard/...d.php?t=851928
To be honest when I saw that post I wondered why someone would bother, but I can see it could fit your needs and answer the questions cheaper than buying the wrong size rims
To go with it, I'd head down the local tire shop, tell them what you are doing and ask if you can borrow a few tires in different sizes from their pile of used tires waiting for recycling
You could do the same thing with a tape measure, plumb bob and googling tire sizes. Not as visual and you'd order your new rims and tires with some trepidation