Brief update: Replaced one of the screws for the extruder mount with a shorter one so it can get out of the way of the hotend fan. Boy, this is tight design. All things wire up nicely and run, the new hotend heats quickly and cools off slowly. Unfortunately, and I should have checked this before, the hobbed part of the hobbed bolt is not where it should be, due to the nut not being far enough into the gear. Melting was not good enough. I will have to dremel.
The fan on the hotend is small but noisy. Handy, though, that it could be wired to the P connector on the breakout board. Hopefully it'll keep the filament from melting too high up.
Update: An overexposed view of the molten mess that is the nut trap of the large gear. There is a fairly obvious extra layer that should be gotten rid of, though I'm not sure it'll be enough. We'll see.