Well, I guess if you really want to do it to the strictest of fire code standards, you could install a 3/4" or better wood "cap" on the top of the stud wall plate extending to the masonry. Other options would be a wider top plate. Neither of these options is practical at this point. You could build the stud wall and add solid blocks vertically above the top plate. This would typically be short (14-1/2") pieces of the same material as the floors joist, typically 2x10, toenailed in place. The object is to get a tight fit on the fireblocks, if possible. If not, larger gaps could be filled with the orange fire block spray foam, smaller ones can be caulked with fire rated caulk, which is typically a red, gritty, fire rated material that is available in standard tubes at the bog box stores. Or, like 99% of installations like this, you could just ignore the issue. I'm not advocating that you do so, but it's a detail that is typically ignored, even by a lot of inspectors.