Engine starts but dies when ignition key is released

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I recently repaired my transmission on my 1990 2.9L Ford Ranger 4x4 Manual after one of the synchro sliders broke. I've been able to reinstall the transmission but after that repair I can't get my engine to stay alive. I've turned the key to Start position and I'd can hear the starter kicks in and the engine starts up, but when I release the key and it returns to Run the engine will instantly die.

I've purchased and installed new battery, and every forum I've seen post advises to get a new ignition switch so I've replaced that too but my problem still persists.

Notably, while holding the key in the Start position, I can rev the engine so it seems like it's getting Fuel, Spark and Air but I don't know if that mixture changes when the key is released. I've uploaded a video the engine starting, it dying when I release the key, and me restarting it to giving it some revs. On the second start you can hear the Starter reengaging because I have to maintain hold of the key

I'd love any input on why this might be happening. I suppose I could've installed the new ignition switch incorrectly, but that doesn't feel likely to me. Any advice helps!

 

[SenorNoob]

Unless you rewired something while doing the transmission, I can't see how the two could be related.

 

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I totally agree, they should be largely unrelated (I guess starter does affects ignition). Still, pre-transmission-removal out was the last time the engine was running properly.