Installed the engine/trans, exhaust, all accessories, driveshaft, trans cooler lines, starter, and engine harnesses. Built the engine cage a bit too close and the power steering reservoir cap was impossible to remove. Grind off two mounting bosses, tilt reservoir slightly, and drill/tap two new holes. Problem solved.
The stock Explorer exhaust is the lowest point on the truck by far, so I fully expect to land hard and annihilate it at some point.
Extended the fuel pump/sender wiring, ran a battery cable from the engine bay fusebox to the battery switch in the cab, ran the main ground from the batteries up to the starter, and assembled the necessary interior bits to test some things.
With the electrical system 90% intact, I threw 7 gallons in the fuel cell and primed the system to check for leaks. Lucked out and not a drop. I tried to turn the engine over, but no go. Looks like all of the critical bits are connected, but eh I'll keep assembling and assume the issue gets resolved along the way.
Edit: Now that I look at the first picture, the ground from the engine bay fusebox is flapping in the wind. Kinda need that if you expect shit to work. Regardless, this pile should be running and driving tomorrow night.