Get yourself a worm corer. Use it to pull a plug of plastic from the soft body on the underside away from the direction it wants to swim and stuff the appropriate sized weight in the hole. You can use lead nails, but an errant running Castaic out of the box will typically require more than nails. Try using the pencil weights you'd stuff with rubber for a Carolina rig. Or they sell pencil lead in coils. You can true up the Castaic, but it will never run as good as one that runs true out of the box. You can get it to run true on a slow grind, but probably not on a fast grinder. I have about 5 of these lures and one that runs like crap. Mickey (Mission Fish) did this type of tuning while we were fishing LMV and got it to run straight on a slow grind.