I want to install a 32" door (I have the slab) in this 36" opening between the finished basement hallway and the unfinished furnace room. So I'm thinking I'll frame the doorway to fit by adding a couple king studs to narrow the frame. I understand I'd need to nail some sole plates to the concrete floor, then toe nail the studs to the sole plates and head plates. Problem is the existing basement subfloor seems to run halfway through the door opening. I think this is plywood or OSB over DMX. So I'm not sure if I should A) attempt to remove the subfloor in the way, or B) try to frame around the floor. If A, then should I try and cut in place or do I need to remove a large portion of the subfloor and reinstall up to the wall? If B) do I cut my sole plates in half width and butt them up against the subfloor? Or should I just cut a 1x1 notch out of the end of a stud and then run that stud right from the concrete floor up to the top plate? Pictures attached. Really hoping you get this and thanks for the help!