Do you use a normal 1040 for a deceased parent? They hadn't had to file because he was 91, there was one SSI & 401 check but the SSI check was taken back.

Yes, you would use the normal 1040 to file your Dad's return for 2023 and 2024, if it is required to be filed.  If there was only social security and pension he may not have to file a return for either year.  The filing requirement for 2023, for single and over 65 is $15,700.  Social security may not be taxable but here is how it is determined for a full year.

• if half of your social security, combined with your other taxable income is below $25,000 for single ($32,000 for married, or $0 for married filing separate), then none of your social security would count as taxable income.  
• IRS FAQ for Social Security Income

If he only received one pension payment in 2024, then he would be below the filing requirement for his return due April 15, 2025.  

Sending my sympathies to you and your family.