Starter engages but doesn't spin?

[FourtyKid]

VH45 swap, Z32 TT transmission and Z32 TT starter.

I just finished hooking up my ignition switch panel. When I hit the starter button, I hear a loud clunk from the starter, which I'm assuming is the starter teeth shooting forward so they engage the flywheel, but the teeth don't spin at all. I've taken the starter out and tested it with a battery and jumper cables and it spins like a champ.

I figured it was a grounding issue so I got a 12" ground cable and ran it from one of the starter mounting bolts and bolted the other end to the frame rail. Still does the same single clunk.

It clunks once when you press the button (shoots out to engage) and once when you release (retracting back into the starter) but it just doesn't spin. Everything else electrical works except the starter.

Note: My battery is pretty much shot so I'm using a battery with a booster pack on it which gives it plenty of juice. Is this my issue? Do I just need to buy a new battery?