She more than likely tosses her head because she's enjoying the run and doesn't want to stop. My old mare was also difficult to stop when we're doing cross country, but I circled her and she would come back to a canter and slow down. Can you circle after you're done racing? Or are you on an inclosed trail? If circling's possible, it's your best friend. If I couldn't circle, I would just give a line of strong one reined half halts while holding the other rein (not pulling it, but a good hold). This is essentially a one reined stop, without the circling aspect. Could I suggest a running martingale instead of a tie down? A running martingale attatches to the reins, so the horse has more of an elastic feel when he tosses his head instead of a dead constriction like in the tie down.
I know what you mean about the different bits not working when you're racing. When I did cross country, after the course it wouldn't matter what bit I had in her mouth, the only thing that could stop her was circling. I, btw, used to use a twist when jumping and doing cross country. She would drag to fences with a regular bit, but as soon as I put a twist in her mouth she became light (for good reason). This gave me the chance to train her to come up from my leg an seat, and if she didn't listen to that, apply it with a half halt which would make her come up. I soon could jump her with my regular bit, and she was softer than she had been with the twist.