Check your fuses in the fuse box and the fusible links near your Glow plug relay. I had the same problem where everything worked in RUN but when I turned the key to START, nothing happened except smoke from under the hood.
Turns our the guy before me, with the old crappy relay, had busted the relay and popped a fuse (very bottom right of fuse box 30 amp fuse), and then bypassed the starter relay circuit with a push button starter, and remove the fuse leaving nothing there. After doing the doghead mod and an ignition switch replacement, I was still running all that current through the fusible links rather than the fusible links and the fuse, so the fusible links melted. Replaced that section of fusible link, popped in a new 30 fuse, and my truck cranked right up after sitting for a month of confusion and being out of town
Good luck, this may not be the issue but at least its another thing to rule out!