Also, you’re the first to mention the exhaust. We added a muffler that sits near the middle of the car but, the exhaust sounds being emitted are a lot louder than they should be for this 318. The exhaust manifolds all seem fine but, further down pipe is suspect.
One possibility: on a later '70s engine the heads have small holes under the exhaust ports that originally lined up with air or EGR, I forget which.
Get the shop manual, you'll see where the muffler goes.
Exhaust outlet under the car will sound loud so I wouldn't worry too much about that right now
