81Coupe,
On my '81, I put Alumincote 5000 headers and a replaced the three-stage Cat with a two-stage; 3" pipes back to stock mufflers, removed the AIR pump and plugged up the plumbing. There was a very noticeable increase in torque and accelleration. Also gas mileage went up too. Just ran from 1300 miles round trip lugging my 200lb son with me and w/o AC running got 22.6mpg twice. I will hopefully have an opportunity to put her on a Dyno in September and once and for all get a reading on the exact increase achieved. Oh yeah, still passes the (very tough) MD emmisions test with flying colors. The whole upgrade was under $400 about 10 years ago.
Good luck and Welcome to the CAC. Never hesitate to ask a question, we're all here to help.
....... Nut