I'd beware of using a shield, or specifically that one due to heat issues. It is bound to get hot and the EZ-B already has a couple high amp voltage regulators. The combined heat may be too high. The Sabertooth motor driver series by Dimension Engineering is what we recommend for high amp control.

Or, if you want to stick with an 4 wire style HBridge, than I use this for my snow shoveling robot: http://www.ebay.ca/itm/1xArduino-Smart-Car-50A-Dual-Motor-driver-module-high-power-Hbridge-strong-brake-/261004097423?pt=LH_DefaultDomain_0&hash=item3cc50e778f

But sabertooth is pretty awesome

Thanks! I'll go check out sabertooth.

I have a 12 amp RMS Sabertooth , it's a very nice controller.