Well I'm no expert on this, but your current spring probably gives you extra sag on the rear suspension taking weight off the front end, which should make the steering easier. Wouldn't putting a heavier spring reduce the amount of sag in the rear suspension and putting more weight on the front end?
If you are over 220 pounds, you probably need a heavier spring regardless of your steering issues.
Most people adjust the front shocks to reduce steering effort, or the adjust the limiter strap.