I found this online....
http://www.food.com/recipe/authentic-shoo-fly-pie-straight-from-lancaster-co-163320
As for the sweetness, I agree about there being a lot of molasses, so I don't know of any way that you would be able to lessen the sweetness, unless you added another ingredient.
To help cut the "sweet" in the pie, you might try adding
one of these to the mixture--
2 tablespoons of Apple Cider Vinegar
1/2 cup chopped, salted peanuts
1 cup crushed up salted pretzels (this would change the texture of the pie)
Or you can serve it with a sour cream/whipped cream topping, vanilla bean ice cream, or something like that.
Cooking with molasses is tricky, but once you understand how it works when cooking and baking, you will know how to use it the way you need too.