That is one crazy ride, nice job.
I think you would have better luck moving the air up from the bottom rather than pulling it down with the fans. Can you mount your fans and shroud up top? or reverse polarity? hot air rises. Maybe you could fab some air deflectors to direct cool air from under the chassis up towards the bottom of the rad and that would help the air flow move up through the rad along with the fans. The rad will cool better with up flow IMHO