The knuckles an hubs PN makes are for the old ZX-5. They will not work on the SP. I had some of their knuckles on the front and they bent rather easily. But that was before they started using 7075. I used their caster blocks as well and they worked nicely. You should be good with the 4mm towers. I just picked up a set myself and they are beefy.
As for the Diff gears. Shim the diffs so the mesh is a bit tight when first built . When you run the car they will wear in and free up the drivetrain. Don't go too tight on the slipper or it will eat diff gears like its goin out of style.
Brad
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