A battle is brewing in Detroit over the country’s largest office conversion project, one that is testing taxpayer appetite for using public money to revive city business districts. General Motors and billionaire developer Daniel Gilbert are planning to overhaul downtown Detroit’s iconic office complex Renaissance Center. The venture’s $1.6 billion plan would demolish two of RenCen’s five towers and retail podium in a 5.5 million square foot complex that is disconnected from the rest of downtown.