If you're not doing animation, you can get M3 clothes to fit well enough on the default David. I got the M3 Safari shirt to conform and fit David just with scaling and tweaking - manipulating the Side-Side and Bend dials by a few degrees. (There's no postwork on this image, and yes, the shirt is conformed to David, not parented.) When you pose David, you'll probably have to do a little tweaking on the clothing, but it can work. Obviously David clothing will fit David better, but the M3 wardrobe isn't totally wasted on David.