If the person being sponsored is a spouse, or son/daughter (who is 18 years or older) of a U.S. citizen: The minimum cash value of assets must be three times the difference between the sponsor’s household income and 125% of the federal poverty guide line for the household.
Not sure about the 401K. Is this a US asset? Is it readily available to be liquidated if necessary?
Just as an aside....... notice that your next visa application is coming up FLR(M). Are you going to apply for that or let it lapse?
I ask because, if you decide to apply for a US visa for your husband, that would take 5 to 6 months minimum (if filed at the US Embassy London)