The mid-1970s were kind of an odd time for Chevy's Corvette. On one hand, the early emissions-era engines decimated the Corvette's performance. On the other, this era did represent a high point for the sharkiest Corvette styling. In 1975, GM finally began adapting to the realities of emissions regulations, introducing a new "High Energy Ignition" that allowed for better emissions and fuel economy while leaving room for serious performance down the line.
The HEI system has three kinds of timing advance. The base or "initial advance" is the physical position of the distributor, adjustable by turning the distributor. "Centrifugal" advance changes the timing as rpm rise. The "vacuum advance" system adds or subtracts ignition advance according to engine load, which helps with fuel economy and emissions.
The conservative factory settings in the HEI system were geared far more toward emissions and fuel economy than performance. The factory base timing was in the 4-to-6 degrees of advance range; however, 12 to 16 degrees of base timing will give you the best performance. The maximum centrifugal advance from the factory is about 15 to 18 degrees near redline with the vacuum advance disconnected. However, the 350 really works best with 36 degrees of total advance at the engine's torque peak -- about 2,500 rpm in this case. You can correct this problem using lighter aftermarket advance springs, which will bring the centrifugal timing in to a full 26 extra degrees at 2,500 rpm.
The stock vacuum advance will ad as much as 16 degrees more advance, bringing advance up to about 52 degrees at part-throttle cruise, which works well. However, it will add too much timing under wide-open throttle if you set the base and centrifugal timing for performance. If you change the base and centrifugal advance for performance, you'll need a different vacuum advance unit that will slow down advance and limit it to about 8 degrees under full throttle.