Sounds like it could be anything. But, if things suddenly break right after I've made some changes, I usually start there...
I had a self imposed problem like this one time when I was trying to install an anti-theft relay to interrupt power to the coil. In the process of wiring up the relay, I only had it powered when the ignition was "on". It didn't occur to me until after a lot of cranking that there were separate voltage circuits for "crank" and "ignition on" in the car I was modifying. Wondering if any of your turn signal wiring and/or 12v conversion wiring might be causing an issue where you get voltage to the coil with the ignition on but not while the engine is cranking? You could try connecting a test light or voltmeter to the positive side of the coil and see if the voltage changes when cranking.
I also work alone most of the time and need to come up with some creative ways to get things done. In order to effectively verify spark to a plug, I've connected a timing light to the wire while pointing the light to somewhere I can see it from the cab. This might help you verify you're getting spark while cranking the engine. I also have long leads on a couple voltmeters that generally allow me to put the meter somewhere I can see it while in the cab of the truck.
