I was at about the same point in my career as you when I figured it out. I matched well, not superbly but at a great program that really fit my needs. I knew I wanted to do something surgical, but I was thinking CT vs plastics vs ortho for the majority of my third year. I had chosen ortho and didn’t decide on NS until late feb/early march.
It can certainly be done. You need to talk to your chairman and PD (if not the same person) before you do anything else. Let them know what your plans are, and be ready to answer questions about why it took you so long to figure it out. You’re clearly enthusiastic; when they see that, they’ll be in your corner.
Have your CV ready before the interview. Be ready to go over it, and make an honest assessment about where you should apply. Talk about where you should do away electives. If your step 1 score is above average, do not take step 2 until later in the year; it can only hurt you at this point, and even a 250 won’t add that much to your package.
You’re in good shape, and you should relax, but not too much. Your enthusiasm and interest will make up for your late entrance on the scene. Best of luck!