Vacuum leak??

[HotRoddin]

Ditto what Rev said .... I would pay careful attention to the intake manifold gasket, especially in that engine, they had a tendency to spit them out. Replace the intake manifold gasket and the carb to manifold gasket, those are two very cheap very easy fixes on your engine and you will have ruled out 60% of the things that could be causing your problem.
In the old days to check for a leak in the intake manifold gasket they would take an oil can and squirt heavy oil along the top edge of the intake manifold ... if there was a small leak there, the oil would seal it for a second and the idle would noticably change ... if the leak were large you could see it suck the oil down into the intake port ... its messy and sometimes smokey but it does work. Another way is to take a length of 1/4" or so rubber fuel line, put one end in your ear and run the other along all the gasket edges where you suspect a leak ... the tubing picks up only the sound very near its tip, so when you get near a vacuum leak you'll hear the hiss like its amplified ... that one isn't so messy but you do look like a goofball with the tubing in your ear