A few things to check. Sorry if this is basic, but not sure of your experience level or what all you have on the engine.
Distributor - if it is points, have you checked the dwell? Made sure the rotor/cap/condensor are all good? If HEI, you certainly don't have to worry about that.
Timing - Set RPM below 1000 as Geezer mentioned. I generally try to not be above 650-700. Remove the vacuum line to the distributor and plug it. Make sure the engine is warmed up and not on fast idle or the choke engaged.
If you are having trouble dropping the RPM, then likely you have either a vacuum or carb issue. Others can surely describe troubleshooting a choke better than I can. Not my area of expertise. Can usually get one running pretty good, but it is trial and error.
For chasing vacuum leaks, of course look for any cracked or missing hoses. For a leak say at the intake gasket or carb gasket - with the engine running, take some wd-40 and spray the gaskets under the carb and along where the intake bolts to the heads. It doesn't take much. If the engine smooths out, you just found your leak. If you have power brakes, take the vacuum line off the booster and plug it as well. Check that the line to the vacuum modulator on the trans hasn't slipped off or is broken.
That is all I can think of right now. Still early here in Denver...
Good Luck...