Horses: specific horse shoulder muscle widening for halter class? Please read...?

My medicine hat paint is not halter stock, but he'll hold his own in a halter class and we'll usually pin.

This year I am chasing a high point award at a certain series and halter class points do count. So I'd like to get out of the 4th- 6th range and pin just a little better.

Conformation wise, again, he's correct enough to pin everywhere we go.

His coloring, though, makes his shoulders look more narrow than they really are. He's white and has a dark patch in the center of his chest (a "shield" it's called). But it kind of makes his shoulders, which are not extra-wide-halter-stock to begin with, seem more narrow than they are. Because it makes this optical illusion of a narrow chest. Like his chest is the dark spot and his barrel is white. But it's not, his white shoulders go all the way out to almost his rib width. He is fairly correct, just the coloring makes it hard.

So, what kind of exercises can I do with an average-width chest stock horse? Or grooming hints?

We do lots of lateral work every day each way, and we do lots poles on the ground and such.

Any thoughts...