Some others have done the neck plug transplant, though it seem like it would be difficult. I'm guessing you could also try to drill the hole slightly larger than what you want and apply some silicone gel around the edges until it gets to a size that will allow the neck plug to fit as tightly as you need it to.
With customs, I find it is easiest if you use the sculpts with necks attached. Then, easy enough to find one of the little neck joints you need, cut off the part that plugs into the stock heads, then glue the custom in its place. Unfortunately, that's not an option with muscle bodies.