Don't over complicate the weighting of a resin lure. The only differences from a resin lure to a wooden lure is buoyancy and the need for sealing. Yes, you can absolutely drop bird shot into your molds, and you will often succeed but when in doubt you can always just drill a lead hole into your lure and fill it with lead and a filler. Don't worry too much about having to drill your pilot holes through lead, you just have to be careful and take your time and clear your drill bits, even though drilling in lead is not ideal. Be careful while working with microballoons, resin, and lead. In particular, microballoons and lead are very bad for you. If you can, wear a respirator or ensure that you have very good ventilation.