You'd need to replace the bechemel to eliminate the carbs in the flour. You can make a similar sauce with butter, cream cheese, heavy cream, chicken broth, and an egg yolk. It will taste a bit cheesier than typical beschemel, but that is hardly problem for mac n cheese. And the wheat pasta needs to be changed out, obviously. You could always get low carb pasta, or just replace the mac entirely. The sauce would go great on some steamed/roasted veggies too. Like broccoli or cauliflower, green beans, mushrooms, etc.