It will depend a bit on what you want to do with the exhaust. If you're planning to replace everything anyways and have the money to do it, replacing the exhaust will be easier to deal with while the back end is out. That said, the exhaust can be changed out even after the car is fully assembled because it can go back in small pieces that are clamped or welded together, so it's not something you have to do right now if you're trying to conserve $ where you can. I always laugh at myself when I go back to when I started my project because I was only going to change out a cracked dash pad. One thing led to another and now my car is completely gutted. It should work out in the end, but I've been telling my son repeatedly that he should limit the tear downs on his project car to just a couple of systems at a time or he will end with parts everywhere.
One thing I would suggest you consider doing while the back end is out is dropping the gas tank. Between the dry rot on the rubber fuel line, a level float that had a hole in it such that it was more of "sink", cracks in the rubber grommet where the fill pipe goes into the tank, and a pile of rust scale sitting on the bottom of the tank just waiting to plug any and all fuel filters, everything needed to be replaced. I bought the replacement stuff as a kit from Roseville and would readily recommend it. Post another note if you have any questions on handling any gas fumes or leftover gas.
