X3 and Super X3 are very similar. I posted my plans for my X3 conversion on my website (see sig line) - feel free to look them over and look in the photo gallery for how I fit the Z stepper back there. 425's will be fine on X and Y direct drive. On my Z I have a 640 with a 2.6:1 gear reduction making my Z about 1664 oz/in. You do need more force than the 425 - but I don't know exactly how much. I can make my Z stall if I want to. The power supply is on the small side - I run 52V and really should run higher.
Oops - 2mm pitch...you're running the ACME leadscrews? If so you won't have speed, and the efficiency of the screws is going to sap a lot of the power. Not sure what to recommend. Ballscrews were a definite for me from the get go. Still - the concept of the motor mounts on my plans could be adapted to the stock ACME screws, you'll just need to do some measuring to locate the centerlines.
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