I know this might be a little late, but...
When I installed my Gardner system, I only used hand tools, including a torque wrench. No need for air tools.
If the original tail pipe hangers were attached with screws, that's what I would keep. No need to install riv-nuts if the screws are holding.
The hangers in front of the muffler install using the 2 holes on each side where the small L-shaped brackets are now. They are relatively tall hangers, and they drop down & pick up the pipes that go into the muffler.
I don't know your location, but I have a set of 4 new 'correct' reproduction bolts for the front hangers and a set of nice shape repro front hangers left over from my exhaust install - PM me if you're interested.