Cooler
Brad's U cooler would replace the stock coolers that you have and be probably the best option. I have a similar set up on my ProX with a 660. If you want to go less money, you may be able to add a rear cooler from an Edge chassis at the rear of the tunnel and plumb it into the hoses on your current cooler. It just revets to the bottom of the tunnel. Add scratchers and it may be enough to cool it. Next option would be to do that and add running board edge coolers from like a RMK and remove the stock XC tunnel cooler. May still be marginal becaus the RMK also has a front cross over. I ran a XCR motor in a Gen II RMK chassis and it would run hot until I added scratchers. It had all 4 stock RMK coolers. Front crossover, running board, and rear cooler.
Brad's cooler will be lighter and cool better than the stock RMK stuff.