It depends. it sounds like you are already receiving survivor benefits if you are receiving your ex-husband's pension. Typically you may receive benefits if you were married to your ex-husband for a long enough period (typically 10 years or more).
You may wish to contact the plan administrator for further details if unsure of your eligibility in receiving these payments.
There is nothing to claim on your income tax return for being a spouse survivor. You owe income tax on whatever your income happens to be. If your income includes a survivor's pension, that's part of your taxable income. If you think there is a state exemption to tax on the pension, you have to check with your state.
If you are providing care in your home for a qualifying person, usually a child dependent, you can use the filing status of "surviving spouse" (used to be called qualifying widow/widower). This provides some of the advantages of married filing jointly instead of single, but it only applies if you are caring for a dependent.