400 meter dash is just sprinting, I'd assume?
If so, if it were me I'd go for pure strength gains and explosiveness. Muscle mass isn't really an issue for sprinters, I would think. It's really only an issue for sustained long distance shit. I would think you'd want as much power as possible, and just be able to go all out for a relatively short amount of time with quick recovery. Strength training (Starting strength is a great workout for good all around gains, pirate bay the dvd or some shit if you have access to a decent gym) and HIIT cardio (You can do it with lots of shit, just google it) does that. Same shit I do for battling, pretty much. You want to be like a goddamn predator. Cheetahs and shit fucking sprint. Gazelles, or whatever the fuck cheetahs eat, are the long distance dudes. No one wants to be a goddamn gazelle, unless you do marathons or cross-country or whatever.
Makes sense to me. And it works great for manly grappling.