If it doesn't want to idle it's probably either the idle circuit being problematic or the secondarys are sticking open. Also the throttle shaft could be worn out causing a vacuum leak, which is a common rochester problem that is fixable. Check the float level first, then back out the idle jets and clean them out (the carb may have some kind of plug or stop keeping them in, being from '75, but there should be a way to get them out. Then start with them two turns out and adjust them both the same amount until the engine just starts to stumble and then back them out 1/4 turn each. This should give you a pretty good idle.
The quadrajet is actually one of the best factory carbs there is but a lot of people don't understand them very well because they are not as simple as a Holley. They are very good once they are tuned right.
Doug Roe has a good book that is probably in your local library called "Rochester Carburators".
