Starting Issues - new turnkey motor

[jtinpdx]

Hi All,
I'm trying to troubleshoot an issue I'm having. I have a 71 C10 with a new GM turnkey motor in it. Turnkey, so all parts are new including the alternator. Some additional wiring was done with the install, specifically a new radiator with electric fans. New battery too. Truck runs great, but...

Sometimes when I turn the key, I just get a click. Doesn't turn over at all. Seems like it doesn't have the juice to turn the starter, but I don't think that's it. I've had the battery tested and the truck tested for electrical draws. All checks out. The battery gauge reads about in the middle.

But if I put a trickle charger on it, it will start immediately. One thing that seems odd is that if slowly turn the key left (to accessories on), sometimes it will turn the engine over.

Any chance it's just the ignition switch, even though the trickle charger symptom seems odd?

Any ideas would be much appreciated.

Thanks,
Jeff