Capital gains tax on acreage

My husband and I are both retired; over 30 years ago we purchased 40 acres of land for $10,000.00 with the intention of retiring and building a home but our plans changed. We have the property listed through a real estate agent for $75,000.00. We have a buyer who would like to purchase it on a Land Contract with 10% down 5.5% interest rate over a 10 year period. We also own a home. What percentage are we looking at for capital gains taxes? Do we pay over a period of time or do we pay when the final payment is made and the deed is turned over to the buyer? Our combined income is between $32,000.00 - $35,000.00 from S/S and pension.