What do you mean a "Project"? A completed and rendered AVI or what? Stretching an avi will making it slower? Do you mean stretching a Nested project? In any event this will make the item slower.
Or do you mean create w i d e s c r e e n from 4:3? I'm trying to understand. I'm lost?
Select all the events in the project. Press G to group them. Hold the Ctrl key down and stretch the end of the last event out to the right. Everything will stretch out proportionally.