im a MTB racer as well.
Whilst bridging to the top guys implies more then fitness, here is what I think of it:
*workouts are not exclusive*
-You could try to improve your FTP with specific L4 workouts (2x20, 3x15, over/under, 5x5 with short rests). Some may argue a high FTP isnt necessary for MTB racing, I disagree. A high FTP means you have more room to push your L6 and short L5 power up and you have a bigger engine to recover from short hard efforts. It might help staying strong towards the end of the race as well.
-You need a good anaerobic repeatability. I personnaly train with short, sharp efforts with short rests (30/30, 20/40, 15/15, 20/10). I dont aim for absurdly high anaerobic power but really focus on repeatability. From race file analysis, at my weight, the high power efforts are often in the 500-600W range with some short bursts in the 700W range.
Technical skill and pacing are other components that would help you achieve your goal.