Since AR15 and 398ARs use the same diameter receiver extension, there should not be an issue. Law Tactical states that it will work on either format.
One thing to consider is the added weight of the buffer insert and what it does to the recoil and functionality of the rifle. Here is a good breakdown of the weights involved in the Law Tactical folder and different buffer weights.
Just like you found out with your gas port diameter, the buffer system has to be correct as well.