I worked with a Paint like that before, saddle fit her just fine, it was just the fact that she was just completely round, like a barrel with legs and a neck sticking out. The saddle slid around EVERYWHERE. We put on a breast plate/collar and a crupper. I also used a very thin pad, if anything at all, besides the anti slip pad. Usually I actually only rode her with that pad. We also did a lot of trail work (trotting up hills, etc), and that seemed to help her build some better muscle that "held" her saddle better. Hope it helps!
"Don't turn you disabilities into a crutch, turn them into legs and run with them"