Section 54

I think the indexation for 1981 would be 100 and the cost of acquisition would be the market value of land during 1981.

Yes, you should refer Valuation officer to determine the value of land.

Totally agreed with mihir and deepesh, you should go for valuation by valuation officer to determine the value of the land for the year 1981.

Refering an valuation officer is always an available option. However if you do not want to do so you can get the details of the nearby ( adjacent) plot sale in the same area during or near to 1981 and derive the per sqm or per sqft rates of that year. Add other development expenses per sqm/ sqft done by you after that that and calculate your value according to area of your land.