First check if your break fits in with your rear wheel on. If it doesn't, then file down your brake. When the brake fits in with the wheel but the spring doesn't, then put the same spring on the deck with a pro-model brake and no rear wheel. Then push the pro brake down so much that it goes past the dropouts. That way you can bend your spring. Don't stretch it too much though.
Btw, when using a pro spring with a blade break, you might have to cut a little piece off the spring in order to fit a 110mm wheel in the back.