I'm using 7075 for the new motor mount I'm making. I've found that it's a lot stronger than 6061 and isn't that much harder to work. I don't want the mount flexing at all, especially when my buggy is doing cartwheels down the track.

I like the delrin, and it isn't that expensive. Although I may be mistaken, my personal experience is that delrin taps well and really holds the threads.