I used Sealey CP2450 - Cordless Impact Wrench 24v 1/2"Sq Drive 410lb.ft and didn't have much of a problem, the reson why I recomended corded one was due to price, the cordless one is nearly £200, have a look on machine mart webside
http://www.machinemart.co.uk/shop/pr...-impact-wrench
Looking at the spec it should do the job 450nm