hey copteazer, how's it going?
Guys like Dynamoe Joe, Big John and Winter Brew are great clutching resources for the ZX. I would follow thier lead.
What has worked for me on my 2100 mile 03 ZX highmark extreme with stock 159 and stock primary is that I bumped up the gear ratio even higher than the stock 2.26 (19/43) and went with a Team LW 04 secondary with a nice stiff spring Red/Black 140/240 with a low angle 42 degree straight helix. This setup provides some good grip on the belt and minimizes the belt slipping which reduces heat. In addition, the higher gear ratio places less work load on the clutches. The only time I tinker with the primary is to adjust the calibration screw position to tweak RPM depending on snow conditions.
I've also found that keeping everything clean, parallel and properly aligned with proper belt deflection is real important. The torque stop should be properly adjusted. On my Polaris sleds I use the SLP torque arms to supplement the torque stop but I've never installed one on my ZX. I dont know if SLP makes one for the ZX but they are helpful to keep the motor/clutches properly aligned under max load.
Although unrelated to clutching, I lowered the needles one position, jetted down two sizes and went with 2 flo-rites filters in the dash. I left the DPM completely stock and it runs great. I dont know if the 01 ZX had the adjustable needles. If I recall, I also think reed spacers showed up on the 02 or 03 but I dont know if they were on the 01?
It sounds like you have adequate venting but the last item I believe helps is that I have the front hood hinges set up for quick disconnect which is the ultimate in venting cool air to the clutches.
Long story short is that I would check in with Big John and Winter Brew but, if it were me, I would start with the gears and tighten up the secondary.
Haven't smoked a belt in a long time and can't remember not being able to rest my hand on either clutch even after a long pull at Cooper.
Good luck.....the ZX is s great sled with huge potential.
Need to know some particulars like what setup is in your TEAM secondary (GOOD secondary btw, keep it!) Also, what gearing are you running now? As "Go high Fast" said, it will make a huge difference, especially with your change to the new drivers & track. Might be the prob with your belt. Anything done to the motor other than the UB head? What do you weigh, and at what elevation do you ride? 
