I'll take a shot at it (hopefully one of our harness gurus will chime in for you)
Starting at the rear;
Adjust the britchen hanger straps so it sits level and perhaps a tiny bit higher. I think if you raise the back most strap it will fix it.
Move the saddle back off his whithers (maybe it just looks like it is sitting on his whithers in which case disregard this)
lengthen the overcheck (better yet don't use it at all unless you plan to show breed shows)
Lower the shafts a bit and push the cart back so the traces can come into play when the horse is 'in draft' (pulling) Again, maybe they are fine when the horse is moving but it looks like they will not loose their slack and the horse will have to pull with the saddle.
And finally, I'm not sure what the strap closing his mouth is. A flash nose band is not something generally seen in driving and if he needs it to hold his mouth shut on the bit I would advise a) give him more time to adjust to carrying the bit or b) if he has had plenty of experience and still gapes at the bit check for discomfort, does he have wolf teeth? points or hooks? Is the headstall adjusted too tightly? Are there sharp edges or some other reason why the bit is uncomfortable? and finally maybe its not the right bit for him.
Hope all that is helpful, in all I have certainly seen far far worse jobs of harnessing and I didn't see anything glaringly wrong.
