Registration of property purchased in 1987.

My Grandfather purchased a property in 1987 and paid full consideration. However, the property was not registered. We have the Agreement to sell from the seller along with the Power of Attorney. We also took a will from the seller mentioning that portion of property we bought should be transferred to my Grand father. My Grandfather is no more now but has made a will of this property to my Father. We also have a relinqishment deed from my father's brother's and sister on the aforesaid property. We have constructed 3 additional floors on the same property. What is the best way to register the property in my father's name now ? a). Through Agreement to Sell ( which I believe will carry Stamp Duty as per prevailing circle rates) b). Through Will.( which may not have stamp duty implication) Also, if we register through Agreement to Sell, will the seller have any Income tax liability since the Registered Value of the property at prevailing Circle rates is substantially high despite the fact there is no exchange of money now or no fresh income to the seller now ? Can we register each floor separately to avoid/reduce the tax implications ? Any advise would be highly appreciated. regards, Rajesh