This reeks of something electrical to me. When I first read it I was thinking valvetrain but now I don't think so. If you can find another ignition switch and plug it in under the dash I would try it. Something about the voltage drop and dome light dimming has me thinking its either ignition or dash wiring related. Was there any dash cluster-switch wiring that looked a little hinky when you put it together? Someone here will know something. There is a ton of experience around.