Finding a Western to fit a draft shouldn't be too terribly hard, we have two draft crosses at the ranch who take really wide saddles.
I can't really help you much with the draft part, as both my boys are rangy with high withers, but I love love love my Longhorn round skirt barrel. I personally like barrels because of the security...they generally have a deep pocket and good bucking rolls. I was taught a proper Western fit should fit one of these criteria: you can get two fingers between your knee and the bucking rolls or pommel, you can get your palm flat between your...ahem...crotch and the pommel, or a fist between you and the pommel.
I was also advised to avoid buying a new saddle, as it takes time to break them in. All of mine, and all of the ranch's, were used saddled. Sometimes you can find really great deals, too. My longhorn only cost me 150 plus shipping, and it looks like it's barely been ridden...just a few scars on the horn and cantle.
Take a piece of cardboard and do a wither trace of your boy, then start hitting the used tack stores, is my advice. ^_^
"Sit tall in the saddle, hold your head up high. Keep your eyes fixed where the trail meets the sky. And live like you ain't afraid to die...don't be scared, jut enjoy the ride." - Chris LeDoux