derrick is correct in that the axles are the same in both, so you can swap them right over. I have also heard that the truck posi are a different design and are weaker, but I don't know for sure.
One thing is that the 73 Nova housing will bolt right in, but it is a multi-leaf rear (most of them) so you should get the springs also or get new ones.
If you need, I have a used 8.5" posi and 3:23 gear out of a 79 TA for sale, email me if interested.
Colorado Springs, CO
68 Camaro... someday!
71 Chevelle SS