Aluminum will cost a LOT more than Mild Steel... Stainless steel will be less than AL, too.
Not to step on your toes or your thread, but
here is another solution to the 3.0 ISR mod that doesn't include bending pipe. I think it's a rather ingenious setup, too.
You could make pipe's and a kit for this one, if you wanted. I bet they'd sell rather well.
My .02
Dr. Z