Not sure how it'd work in the clam chowder, but whenever I have to sub (because of allergies and lactose intolerance), I just sub soymilk...so maybe just use milk? Or, another idea is maybe make milk with powder so you can control the consistency?

Evaporated Skim milk with a little cornstarch added to it....

I would use Cheffy's suggestion. But I've also used sour cream (or yogurt) for creaminess in these types of recipes.