You can install both. One holds the windshield to the body. The other keeps the windshield upper and lower frames from spreading apart. In the old day when they drove 150 mph in these things the glass would blow out of the windscreen.
If you use a top (soft or hard) it's good to have both of them. If you have the windshield out, put the body mount bracket in now. You can always install the center rod later.
Remember the 4-40 screw lenght for both is a "little" bit longer. Cut screw to fit (file). Check to see if the holes are over you glass. Long screw in Cobra windshield break glass!
Dwight