it does not look like a bad fit. What kind of saddle is it? In some phottos, it looks like it might be a bit crooked, like the view from the cantle. And it's uber stuffed, in the panels, no?
In any case, i'd just ride in it for a bit and see if your horse becomes sore . I know that sounds a bit lame, but since you have no real other option, what more can you do? It does not look like it needs shimming.
The only thing I can't see if is the clearance for the wither bone, on top of the wither bone, is good all they way along. It looks ok on the sides of the wither bone, but cannot tell about the upper part. Can you put your fingers in there and make sure you have enough vertical clearance that wehn you put your weight in the saddle, it won't push down on top of his wither nbone?