Is it permissible to offer in-kind materials instead of money for the public good, if the father willed a specific amount of money without specifying the method of payment?

You must adhere to the father's will and pay the specified amount to the designated party, and it is not permissible to deviate from it, even if you see a greater benefit in doing so. This is in accordance with the Almighty's saying: "So whoever alters it after he has heard it – the sin is only upon those who alter it." And part of your dutifulness to your father is to expedite what he willed.