Well the engine came as a complete assembly when I took delivery. I'm not entirely sure what it's history entails, but It starts very strong which is apparent. Luckily when I pull the fuel lines it does indeed shut itself off, so it's not a runaway.

I'll try to take a video of it next time I gather the nerve to turn the key. I have the injection pump from my seized engine just hanging out in my yard, and it turned on with only 4 cylinders. I'll probably take that one completely apart and swap some of the parts in the one that's installed on my new engine. Also, I'll try turning it on with the shut off valve taken out. I suppose I'll be able to shorten the list of possible reasons this is happening.