Do I need to show the 400k for a Non-O visa based on marriage as an overseas international transfer?

No, not for the conversion into Non Immigrant O visa, than you just need minimum 400.000 Thb on a Thai Bank account on your name only and for the 1 year Extension of Stay based on being married to a Thai citizen it must be in a Thai bank account for at least 2 months prior application date.

If shared bank accounts you need the 400.000 Thb multiplied by nameholders bank account.

I did mine in Maha Sarakham. Was not required to be international.

For 1-year extension based on being married to a Thai, banked money doesn't have to be an international transfer. But the best is to go to your local immigration and ask to be sure.