Tut, how about skeletonizing the buttstock to see how much weight can be saved? I can't remember the name of the British gunmaker who made many guns of this style. Thomas Turner was probably the best known, but there were others. Another trick is to give up the ribs as is done on a few Boss 20 gauges. It's a project gun, right? If you don't do it, I will. I think the target weight should be just under five pounds.