My B2650 ROPS anti rattle nut welded to the rops is definitely a 3/8-16 NC thread. They can easily be confirmed with a thread pitch gauge whether it's metric or standard thread.
Regarding door height: My 8 foot OH door into my heated shop would just barely allow my rops to clear when entering with maybe a 1/4" to spare. But when building my winter cab I also wanted it to be able to drive into the same shop with the tractor and didn't want to build the cab under the existing rops. Also, the factory rops wasn't parallel to the ground and I wanted the cab to rest squarely on its top. Both problems solved by making a new top half of the rops, albeit about 3-4" shorter and with the top cross bar parallel to the ground for the cab to rest on. It also mounts and pivots just like the factory piece. I just leave it on year round and don't worry about door clearance anymore. And it still leaves plenty of clearance or height above the head. Maybe doing the same could be the easier way out for your door clearance issues.
I believe those "door tracks" mentioned are called "Low Clearance" tracks or short radius tracks. I used them in one shed to put in a 12.5 foot OH door under a 14 foot ceiling.
