It is only an upgrade to the ZX if you get a 2000 or newer. The 1999 still had the SC10 rear suspension. It would not be worth trading a S chassis with a bulletproof rotary valve 583 for a 99 with the same SC10 rear suspension. JMO
The 600 zx is far more responsive, I've seen these 2 sleds race many times (583 & 600) My buddy had a mxz 583, my other buddy still has his 99 600. The 600 will beat the 583 any time. Especially off the line. Like the others said don't get one unless it's a 2000 or newer.
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