Ltcg on transfer of right for redevelopment to builder

Dear Sir,

Assessee became owner of Residential Property which is taken on Lease for 54 Years (From 1950 to 2004) from Bombay Municipal Corporation(BMC). Now BMC transfers ownership in favour of Assessee few years back.

And Assessee transfer the right for Redevelopment in favour of Builder. Builder will pay 1 New Flat and Some Monetary consideration for it.

Now what shall be the cost of asset for Assessee while determining LTCG for Monetary Value received ?

Market Value as on 01.04.1981 can be considered as Cost of the asset for Indexation purporse ?

Thnaking You.