You can still add an extra beam over the ceiling if you're going to screw panels, so you can see if it fits with 60cc and add a beam directly for them too...
You still need to add another stud over the ceiling if you're going to screw boards, so you can see if 60cc fits and place a stud directly for those too...
I guess he's saying that you may need closer spacing between ceiling joists if you're going to fasten boards as a ceiling, and that while fixing the walls, you should add joists between each rafter.
Personally, I would have solved it with a sparse panel, perhaps 28x70 c/c400, across the rafters and screwed a ceiling into it.