Bob's comments make sense to me too. I would wager that if you shimmed that tree up at the back, it would further exaggerate the problem areas of the fit. I could really seeing the front of the bars digging into that horse's shoulder. A narrower gullet with a bit more flare at the front of those bars and it'd be a better fit.
Have you seen Rod and Denise Nikkel's page on 'Measuring Horses for Fit'? I think their trees run about $600, which is probably not the cheapest option around. But, if you're having a bit of trouble finding something for this horse, I suspect you could take your measurements to Rod and he'd make a really nice tree for you.
http://bit.ly/ZThfR0
BTW, I like your grey horse. Nice red dog too. 8-)
Good luck.