If I were to extend a crankset I would probally avoid a process that required redrilling and tapping, the problem is the angle needed is too much set up time to be worth it, we all know how wonky a pedal is if its bent a little, right? If you get that hole a little off youre screwed. I would be apt to pick up two cheap sets and make one set out of them, one joint, maybe cut at a long angle so there is more weld surface and alignment is easier, you could make a 300mm set out of two pairs of 170's if you wanted this way.
heck, give it a try, It may work out great or it may seem like a custom set is cheap. Try anyway, you never know.