While as Les (LK) stated above, the only "approved" means of copper to aluminum pigtailing is the AMP system, AMP has placed the cost of their system in the realm of a 2 or 3 year old work truck. I recommend visiting
www.cpsc.gov and searching "aluminum wiring" for an official stance on various pigtailing methods.
Just as an FYI, I've seen the purple Ideal wirenuts listed for AL/CU terminations fail just as miserably as a common wirenut would...(maybe about 4 or 5 times) Aluminum wiring of that era is very unforgiving when it comes to terminations, as compared to copper.
