Disconnect the vacuum advance hose and plug the vacuum source on the carburetor. Adjust the idle speed screw down as low as possible and check the timing. The initial timing advance should be 10 - 12 crank degrees BTDC at 650 - 800 RPM. After the initial timing is set, reconnect the vacuum advance hose and you are done.
The full timing advance is built into the distributor by the centrifugal (mechanical) advance mechanism and should be a total 34 - 36 degrees BTDC above 2000 RPM. The vacuum "advance" is actually a timing limiting device in order to prevent the mechanical advance from advancing too much or too fast. The amount of advance allowable by the vacuum advance device is stamped on the vacuum advance mounting plate and is 12 cam degrees. 12 cam degrees x 2 = 24 crank degrees mechanical advance. Therefore: 10 degrees initial advance + 24 degrees mechanical advance = 34 crank degrees total advance.
If you are seeing 60 crank degrees advance, with the vacuum advance mechanism is still connected to a vacuum source, the vacuum advance mechanism is bad, or the weight ring on the harmonic damper has slipped. I have worked on Chevy and Pontiac engines since 1962 and I have never seen a weight ring that has slipped. Just lucky or I don't abuse them.