Behold, it has an interior now! I rerouted the speedometer cable and got it hooked up to a hybrid of the sending unit from the automatic and the gear from the T5's sender. As a result the speedometer is accurate now! And far, far quieter than it used to be as well.
No lower shift boot? Make one out of butyle sound deadener. Interior is significantly quieter now.
Michaela mentioned the clutch was a bit heavy so I crawled under and saw this glorious clearance between the exhaust and clutch fork. A bit of massaging with my angle grinder and the clutch is now fairly light and usable. Certainly leagues better than it was.
I was just about to leave and I noticed this:
Turns out the plunger-style headlight switch these Mustangs have is notorious for getting warm and melting and the wires pop out of the plastic plug that goes into the switch. A while back I had to pop these wires back into place and it appears I somehow ended up putting the headlights on the dome light circuit. A few minutes of googling for obscure pinouts and I got the wires swapped back around and all was well. On the way home Michaela beat a V6 early 4th gen Camaro in a race from a red light somehow. Then a Honda Odyssey trashed both of us so... yay malaise sports cars.