Sold my house in 2024 and I don't know what my deceased husband paid for it in 1979. What do I put for original cost?

it may not matter.  how much did you sell it for?  if you sold it within 24 months of his passing, then your capital gains exclusion is $500,000.    So if the selling price was less than $500,000, it won't matter what figure you use. 

Let's start there.  

You will want to look for tax records, valuations or other records- I would hire someone who does historical valuations- because if you do not the information, your basis will be zero and you may have to pay on the entire amount.  Note that the exclusion for the sale of your primary home may apply as well.  You would also have a step up in basis for his half to the value at the time of death. 

Were you on the deed prior to his passing?  If so, half the cost basis "steps up" to half the fair market value (FMV)  on the date of death.  If you live in a community property state (Arizona, California, Idaho, Louisiana, Nevada, New Mexico, Texas, Washington or Wisconsin), the whole cost basis steps on the DOD. 

Alaska, Florida, Kentucky, South Dakota, and Tennessee, have passed elective Community

Property Laws.

Then, as others have said, you almost certainly qualify for the home sale exclusion.