Alan,
It will be hard to recommend a rig that works with your hunting pack without knowing how it fits you. If the belt is wide and covers a good portion of your waist, adding a gunbelt won't be easy. If you have the room, you can position your gunbelt below the beltline of your pants. Crossdraw would be my first choice when in the field. This allows you to carry a backpack and not constantly bump the butt of you pistol against the pack and allows you to protect the gun with your hands when traversing through thick brush. A flap holster would do that work for you but they're hard to find these days.
Butt packs work great as long as you can position the belt for the pack in a manner that allows you comfort. You could do any with the gunbelt and carry on your pants belt(use a wide, double thinkness belt to support the weight of the gun)and run the pack belt under your holster.
If you give some more information, maybe a picture of yourself wearing the pack posted here, maybe we can throw some more ideas your way.
So hows that big Ruger shoot anyway?