Where I am, the replacement cost of existing houses is far in
excess of their market value. This leads to the following
reasoning. If the house is an investment, then insure for the
(lower) full market value. If the house is your home and you
would rebuild it if it were destroyed, then pick your target
replacement cost and insure for that much. The premium is
based on the potential payout from the insurance policy, not
on what the house may be worth.

Re the OP's question, "How would I get a mortgage over the
amount covered by flood insurance?", it seems to me the trick
is to avoid being UNDERinsured. So I would ask "How can I
increase my flood insurance to cover the value of my house?"

Una