Just put the cowl on with no engine and keep adding weights inside the cowl until it balances and then weigh the weights. Won't be exact but will get you a good enough idea as to whether or not the engine and mufflers you are thinking about are too heavy for the plane.

Wing loading does't matter as long as your engine is the right weight or lighter than is needed to achieve balance. Its going to be what its going to be unless your willing to start carving weight out of the back end of the plane.