YMMV of course, but not necessarily a dumb idea. I am like you: Used to 170 cranks and unwilling to compromise on our tandem(s). I have access to a small machine shop and have simply "cross-threaded" several cranksets for use on cross-over drive tandems over the years. I have not used Helicoils or inserts. Don't know how heavy one would have to be to experience problems, but I'm about 150 lbs and learned the trick from a slightly heavier/stronger engineer and former frame builder.