I live in the UK, so it's not direcrtly comparable, but I now eat for about £40-£50 ($67-$84) per month. The trick is: I eat very little meat (about 5 days a month), and buy things in season, on offer, or reduced-to-clear, if it's good value. I also tend to cook food in batches and eat it over a week. Now that I switched from prepackaged cereal to rolled oats, absolutely none of my food comes in cardboard boxes or rigid plastic trays, which is a good heuristic. I imagine you already know most of these, since you're spending $143 instead of $400 - but putting it into practice every time you go to the supermarket is what counts.
Mustachianism means identifying what makes you happy, and spending your spare money on that, instead of other things. If $40/month for hockey makes your husband happy, then spend it. If $40/month on a cat makes you happy, then spend it. If eating meat every day makes you happy, then spend on it, and if $87/month on other short term pleasure makes you happy, then spend it. But that's a lot of discretionary spending - each individual one can be justified, but would you really stop being happy if you gave one (or more) of those things up? You don't even have to give them up totally - maybe play half as much hockey, or drop the $5/week each and just do $10/week joint. Also consider other ways of making yourselves happy, which don't require as much spending. Considering your situation - both of you have about average incomes, and student loans, and you have a kid - you can have things you want, but not everything you want.