I’m just spit balling here as I know little about racing, and especially dirt tracking. My thoughts would mainly be fodder for more conversation or the opportunity for those who know more to inject something.
I would imagine that a high stall speed would not be as important as in a drag car because you’re already operating up in its torque region. Also, what about using a lock up converter? I’m really not sure if one could be added to a 904, or could you use a overdrive transmission (though it does add weight) or use an overdrive transmission to gain a lock up, but remove the overdrive and use a regular 904 tail shaft? I don't even know if thats possible.