Have you check the motor mounts for loose bolts?. I would hook a vacuum gauge up and run enough hose so you check while drive. 1. Record idle vacuum at idle. 2. Increase RPM until engine run smooth. Record RPM and vacuum reading. 3. Disconnect distribute hose.Record timing at idle Leave the hose disconnect for the rest of the test. 4. Set RPM to above, Record timing. 5. Drive the car. Record the vacuum reading at cruise. Hold the speed for 5 minutes. Did the vacuum hold steady or did it jump around.? 6. If your ride is a auto disconnect the trans vacuum line and repeat the test on the ride home. 7. If the vacuum was nice and steady then I would bump the timing up and then reset the idle until you see the highest vacuum reading. 8. let us know the vacuum reading.
One thing I forgot to mention. Check the vacuum shift modulator on the trans. See if the vacuum line is flattened, torn or off of the modulator. If the car smokes a bit (white smoke) the modulator could be bad, diaphram ripped open, and it will suck trans fluid into the engine. Also, it will appear as a vacuum leak to the engine. Give it a check.
Check the vacuum reading.Hook it to the port on the carb.If the needle bounces around.You have a valve problem.Its sounds like you have a stuck valve or a valve actuation problem.May be time to pull the covers and have a look see.Good luck.
If it helps, my Ruby (72 Mav 4dr) just got a new vacuum modulator and she's like a different car. Much less shake & rattle, smoother idle, no hesitation off the line.
Make sure it's the port below the throttle blades for manifold vacuum. Any other port, like on the side of the carb, will not be manifold vacuum.