You could get away with a much wider aperture. f/5.6 or f/9. That and a higher ISO would allow you not to have to use the pop up flash. The two things killing these photos are the softness and the hard shadows from the on axis flash.
Batteries for the 430 and a diffuser or bouncing it off a wall or ceiling would make the light a nice large, soft, light source and would really help out on the harshness.
With the 105mm lens, I'm thinking you're pretty close. The pop up flash isn't very powerful and will have a very noticable light fall off. All of the photos except the first have the snake's head almost blown out from over exposure and the rest of the frame either exposedly properly or headed towards being under exposed. Bouncing the 430 would cure this as it would put your light source at range (unless your wall is 2' away from the snake) and you'd get less light fall off.