From the graph I have find, a 50Ah lead battery will be rated at 20Ah when you discharge it at 2C (100A). 100A x 72v is just around 8hp!
So, because the 50Ah battery will cost 600-900$ (100$-150$ each) and because you can only use around 15-20 Ah of those cells, if it' just to do some test, you can maybe keep an eye on small LifePo4 cells.
For example, 26 cells 38120s 10Ah will cost around 400-500$ and 26 cells 40160s 16 Ah will cost around 700-800$.
Maybe this 20Ah: http://www.ev-power.eu/?p=p_46&sName...o4-cells-(3.2v
I used 7 Kg of headway 10 Ah in my Smart fortwo to do some test. That worked great.... for short travels at low speed!
It's just an idea!