Just to clarify a couple of points. A 7-8-9 speed compatible crank works just fine with a 6 speed cluster. I've been using a setup like this for several years. Do NOT whatever you do file the alloy cranks interface with the spindle. Its a precision contact point and you will destroy it if you try. What you need is a shorter BB spindle and its a good idea to measure the chainline you have currently with the alloy crank in order to get an idea of what new length you need to buy.

Anthony