stock 1973 318 V8 starts but won't idle without foot on pedal

UPDATE: I got it to rough idle by changing the idle mixture screw setting. I had to back out the screws 4 1/2 - 5 turns. I checked the timing and it is at TDC like the sticker says it should be. Changing the timing did not smooth out the idle. After a push of the gas pedal, the car will cold start fine and fast idle. After fast idle it will settle into a slow rough idle. It is driveable but it has no power and it feels like I'm towing a mountain. I sprayed quick start around the intake manifold and carburetor looking for a vacuum leak but that was inconclusive.
Here are some answers to questions by some responders: This is not a lean burn system, the car does not have power brakes, the gas tank has been restored/lined by Gas Tank Renu, and we drained the gas and put new in before we started this project.
Does a 318 V8 have plastic teeth on the timing gears? Could the timing chain have slipped? The car has 99,000 miles.
I just completed a front end rebuild and I have an appointment on Thursday to have it aligned. This is a general repair shop and I will have them assess why it runs so poor. Thank you for your help in getting it running well enough to keep the appointment and get it there under its own power.