Won't start? Consider this...
 

I had just finished an E-rod install into my '63 Avanti and all was going well. It started and ran immediately as it should. I did some initial street testing and still all was good (great, actually). I was doing the initial GM-specified 30 mile break-in and at about 22 miles I went to start it and finish my miles when all it would do was fire and die, fire and die... over and over and over...

One of my eye doctors, who I was going to go see when it wouldn't start, told me his Jeep did the same thing (complete with gauge-indicated fuel pressure) because his fuel pump was bad. So, the only thing I hadn't changed was my ol' Kinsler pump. I too had adequate pressure at the gauge installed just before the stock LS3 fuel rail... New pump and it started right up! Wow, simple enough... NOT!

Two days later the weather cleared and I was on my way to run another errand and the same as before! AUGGGHHH!!!! I talked to the friendly, neighborhood LS 'puter guy and he mentioned several things, none of which proved to be it. And, it hadn't fixed itself again, btw... Finally, I got a new ignition switch since the old one was only a bit more than a half-century old but seemed to work fine... UNTIL NOW.

New ignition switch and problem solved!

Just something to keep in mind if ya run across something similar. All the best!