Looks nice, you are doing some great work on this Z.
I did this mod back before anyone made a “kit” for it. Sourced the Toyota 4x4 calipers and 300ZX rotors from the local parts supplier, made a drawing in CAD for the spacers and had a local machine shop cut a pair from 7075 T1 aluminum, picked the longer bolts at the local specialty bolt supplier.
It was on my race Z, so I didn’t use the backing plates, instead custom building aluminum cooling ducts to direct air to the center of the rotors and also to the calipers. I made up my own stainless braided Teflon hoses with 37 degree JIC fittings and all the hard lines from stainless steel tubing.
Works awesome, brakes that last all weekend at a race meet and don’t require bleeding after every track session.