USSR pre 1990 might be a good place to get data from, and they had a very controlled economy, so less to regress out. Only issue would be over estimations of output.

I would also analyze the fall of each empire and the great and mini depressions of them all. I would analyze the current 2008-2013 economic crisis. What is causing it, might be related to what brought about the Industrial Revolution.

Isn't the recurrent investment in capital what explains the increase in eficiency?

I would look at the up down trends throughout history and explain them. We know that Rome was a great economic success, China, Egypt, Babylon and Greece. These are all examples of mini revolutions. Clark is looking at world average GDP. I would not. These mini revolutions are the key. Explain them and claim your Nobel.