Long term capital gain

I purchased an office in 1999 sep for rs 6 lacs, it is not registered in my name. Now I will sell it for rs 35 lacs. The stamp duty valuation is rs 61 lacs. Obviously, the stamp duty value is much more than the real market value, I m in no position to pay capital gain tax even after indexing. What is my tax liability on this deal and how to minimise it. Do I need to re invest in residential property only. I hav no residential property in my name, as on date. Pl advise best tax planning for me. I want to purchase another office with the sale proceeds.