The build thread that you referenced is not a great example to look at in regards to your questions. That car had a complete custom chassis built for it with a lot of unique components and ideas.
When running the IRS on a 1st gen camaro, you can no longer run the stock tank. (you can run the stock tank with the solid axle 9" rear suspension) A custom tank would have to be built. Rick's Tanks or Rock Valley can both provide you with that end of the build for $1,000 - $1,500 depending on your motor's fuel pump requirements.
Here are a few good shots of the exhaust and fuel tank on a 66 Chevelle with the IRS. It is a different car, but similar idea on the tank and exhaust. The customer wanted the exhaust run on the outside of the frame rails on this car. You could also run the exhaust inside the frame rails and make a narrower tank.
