I used 5/16 Kunifer when I did mine, I can't think of a reason why using stainless would be a problem as long as you can bend it.
On the brake drums, you might find that the "shock" method is the best way (or the only way) - belt them sharply with a hammer around the outer surface, which often shocks the shoes away from the drums enough to remove them. Also as it's a Magnum, it will have self-adjusting rear brakes which might not be helping. There's a small hole on the backing plate that allows you to get a screwdriver or similar inside and lift up part of the mechanism to back off the self-adjusters, which might let the shoes move back far enough.