Tech guide shows '71 383 likely came with a timing retard solenoid
http://www.imperialclub.org/Repair/Lit/Master/287/Page11.htmand possibly a solenoid controlled vacuum advance valve.
http://www.imperialclub.org/Repair/Lit/Master/287/Page12.htmIf either of those is stuck in 'retard', then the timing will be late and the throttle will feel sluggish.
You'll have to check your car to see if it has either or both, and if so whether they are working correctly, or at least not retarding the timing.
Those are the only emissions items I see in the guide that could effect throttle response. But they are big ones.
