I have gone back and forth between Warn and OEM hubs on a '78 FJ40 and never needed any kits or adapters.
I find that Warn frequently specifies that these kits are needed, when they are not. I did not need the kit on my '83 GMC when I went from automatic hubs to Warn manuals.
As Dave in Denver said, I suspect it's needed for an odd corner-case, but not for regular old FJs.
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner