First thing is to check that all the new plug leads are actually correctly fitted and pushed completely down on the ends, also make sure that you have not crossed over any of the plug leads!
Then try removing/replacing the spark plug leads one by one with the engine running using insulated pliers or something to hold it, when you remove one you should notice the engine misfire and sound different, this should happen with each plug lead, if one makes no difference then that is the one causing the problem.