First you have to record SMPTE/EBU to one of the tracks of the MT8X make sure to bypass EQs and noise reduction, I believe your XRI machine can generate that, but you can also use the SMPTE generator plugin on Cubase/Nuendo. Try 25FPS as it is lower frequency and easier to be reproduced on the narrow tape, also 30FPS generated from the plugin had problems the last time I tried (decades ago!).
Feed this back to the XRI machine, and set the start time on the machine as well as Tempo, connect MIDI to Atariās MIDI input, set Cubase to lock to MIDI clock (not MTC) and you are good to go. (If the machine can also convert SMPTE/EBU to MTC, you can lock Cubase with MTC, too. But the MIDI clock (24 ppqn accurate) was tighter than MTC (1/4 frame accurate) in most cases.)
However, you perhaps need 2nd MIDI input to record MIDI to Atari (I am not sure the XRI thing had MIDI input to merge MIDI clock with inputs, if so, then itās fine but I think itās unlikely). Maybe itās much easier and better to get either one of, C-Lab/Emagic Unitor-II, Steinberg MIDEX+ or Steinberg SMP-24.
*(Donāt get Unitor-C or Unitor-N, make sure it is called 2 or II. if C or N, you will need another dongle expansion called Combiner. Or forget about Cubase and use C-LAB Notator or Creator 
, UNITOR-N/C works as a dongle to run that software. Also, donāt get stock MIDEX without +, it has no SMPTE input. IMO, in terms of stability, Unitor-2 is the best and perhaps the easiest to find, SMP-24 must be so rare to get hold of. The connection of MIDEX+ to the DMA port is quite dodgy, just one little hit to the desk the atari and MIDEX are placed on can make the system freeze! C-LAB had done a much better job.)
These atari expansions have SMPTE inputs, that can be used to synchronize Cubase to any tape machine (or even DAW! 
). Every professional/semi-professional studio in Europe in the early 90s had one of those, itās easy to find one (Unitor-2).
I will try to get my head around all of this and I understood it (sort of 
).