I've got a 350z with the vq35hr engine hooked up to a 2019 370z transmission and it drives great, however the speedometer, odometer, and abs don't work due to the transmission harness not being compatible with the engine harness any clue on how I could get it to work or if there's any conversion kits? Or would I be better off just swapping back to stock transmission?

 

370Z transmissions have the Revmatch feature. My guess is, you would need to match up a 370Z PCM & harness with the trans for everything to work correctly, but then your engine management would no longer function. I would just switch back to the original 6MT for the HR 350Z. The amount of trouble to get things to work with each other would be more costly and time consuming, and ultimately not worth it. There might be a way to get things to play nice, but this might require an aftermarket PCM like a Haltech and custom programming + wiring, things could easily turn into a hack job and still not achieve the desired outcome.

To my knowledge there are no writeups/documentation on this being done successfully with working factory wiring/gauges.

The stock 6MT for the HR is an excellent transmission. The only issue was with the OEM CSC and there are conversions kits and aftermarket upgrades to address that.
Good Luck!
-Icer