Hi, you would be better advised looking through the Lumia 1320 forum for users experience.
As far as I am aware the Lumia 1320 can run Win10M ok, but different users have different experiences.
If you use Insider on the Release Ring, the same version is fetched as the recent public Upgrade received by these phones (10.0.10586.xxx): Phones Eligible for Windows 10 Mobile Official.
If your phone is not on the supported list, then 10586.xxx (Threshold) is the end of the line. You may as well use Insider and select Production to un-enroll your phone from Insider, but at least you have W10M. WaaS (Windows as a Service) will deliver future updates within Threshold to you without any Insider access required.
For right now, Windows Device Recovery Tool is only flashing 8.1 to models that did not launch with 10 so it is the way to revert if you don't care for W10M: How can I recover/reset/restore my phone software?