Just in case anyone else is in the same situation as me (building the chassis, bought the defender body kit), from Traxxas support on what needs to be done.

To mount the Defender body on the Bronco chassis or the crawler kit chassis, you will need the wheelbase extension kit https://traxxas.com/products/parts/8057

The shock towers and fenders of the kit get taken off completely and replaced with the conversion kit's towers.

Unless the Defender body already has all the attachments like this https://traxxas.com/products/parts/8...-defender-body you will also need all of those parts as well. Here is the exploded view showing all the body attachments. https://traxxas.com/explodedviews/TR...-Body-Assembly

So there you go, the TRX Chassis is the same wheelbase as the Bronco, but shorter than the Defender. Took some research and that seemed to be the case, but from the horse's mouth.