The stock spring that came in that sled was a polaris blue/white with a 120/310 rate, the only solid red spring that comes to my mind is a EPI RED that has a 145/300 rate, the stock weights were 10-58's, my question for you is if your engaging at 4500 right now why do you want a 5000 engagement with a 600 twin???, with that high of engagement your gonna just be blowing the track loose and trenching big time, the pink epi spring is a 185/300 and would make for a much higher and harsher engagement and would be very hard on belts, the twins make good torque down low and don't need that high of a engagement, also at low speeds your motor is gonna be at a higher RPM and will make for worse fuel economy, but if you really want that 5K engagement you could try a EPI silver 165/300, or yellow 165/310, or if using polaris springs a almond red's rate is 165/310, I don't think the polaris almond blue 150/310 will get that 5000 your looking for, but if you could tell us why you want to engage that high it might be helpful, or at least cure my curiosity of why.