A smog pump makes virtually no difference in horsepower. It's a low-pressure vane-type pump, and the load on the engine isn't noticeable whether it's running or not. Other emission-related things that might make a difference are carburetor jetting, and the centrifugal advance curve in the distributor. Some models also eliminated the vacuum advance, unless the coolant temperature got high, and a thermal/vacuum switch allowed vacuum advance to come in and cool the engine down. I did a lot of emission-related diagnostics during the 60's and 70's, and it was a real challenge to make the engines perform. Most "improvements" people tried by removing emission controls actually made them run worse, and get lousy gas mileage! It was necessary to do internal carburetor modifications, and recurve the advance systems in the distributor to make any noticeable improvements in power or mileage.
Jerry
