Its a bit tricky to use 4 piston caliper with usual wheel, as you mentioned you need to use spacer under disc for clearance. I did that.
Good to know that original YZ brake is working ok with 320mm disc.
If I may ask which year of YZ fork have you used?
I used YZ250F 2008 and had problem with broken free pistons in cartridge.

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Yamaha XT 660 Z Tenere