You should make a "mango wine" and blend it with your Hefeweizen after fermentation. That way you can control the flavor better. You may need 3-5 lb's per gallon. I know that it takes about 5 lbs to make one gallon of wine, but your only flavoring a beer and you don't want to over power the beer. You may need to sweeten it after blending. Either way, making the base wine separately will give you more control.
In regards to Simcoe as a hop, remember it has a mint flavor and I'm not sure it is appropriate for a mango flavor. Just my thoughts.