Gary,
The metal drive gear has 6 teeth and you won't be able to change from that one unless you change the entire output shaft (not necessary).
With a 6 tooth drive gear, 3.73 gears and 32" tires, you would need 14.09 teeth on the driven gear. With your 1.2 ratio adapter it will help correct some, but I think it will still read slow on the speedometer.
Here's a thread that might be helpful for you to figure out the exact ratio. I can't open the spreadsheet on my phone so I'm not sure what it is, but seems helpful by other posts.
https://67-72chevytrucks.com/vboard/...d.php?t=656949
Keep in mind the math to figure everything out exactly is possible, but there could be variables not reflected in the math vs real world driving. If it were me, I'd wait to see how close it reads once it's road worthy and go from there. Even though gps is not perfect, it's still probably the best way to establish a baseline to calculate unless you took it to the trans shop that makes the gear ratio boxes.
If you're not able to use the spreadsheet, the author seems like the go to source to help you determine what ratio box you need.
If you are able to determine the ratio box you need let me know and I'll take a look at what I have, but waiting until you're able to drive it is also a legitimate option too.