If you have found a pair of 326 link or straight through boards in your NAC72, these are actually quite rare to find and worth hanging onto, even if you don’t use them immediately. Might be useful later, ( if your wish to use an external phono stage, or maybe change that input into line level, for another audio source).
As explained by others, you need the 322 phono cards for MM cartridges. These pop up for sale from time time. If you already know Darran at Class A, best to start there.
Good luck with your 72-140 → 72-HC-140 journey. It’s a fine system 
There are lots of threads and posts that might help you within this forum - most can be found in the Naim Olive thread - the search function is your friend for this.
You may have already seen this, but in case it’s helpful for you…
Best wishes
R