Whether a cap works or not is mostly dependent on your ignition system. If you are running points you will need a much larger capacitor than if you were running an HEI. Even then it is iffy.
The current draw of the ignition system is not constant. When the coil is charging it will pull 4 to 5 amps. That is as much as a high beam headlight. The amount of time that the coil is charging is the dwell. At low engine speeds, the stator will not put out enough power to meet the peak demand of the ignition and the voltage will sag.
The purpose of the capacitor is to charge up when the ignition is not charging. This stored energy is then used to assist the stator when peak demand increases. The cap will quickly drain and voltage will still sag but not as bad as it would if the cap was not present.
The problem with the points system is the high dwell at low speeds. The dwell is 90° and the engine fires every 180° (we're talking breaker cam degrees). So the duty cycle is 50%. At 1200rpm the points system is drawing over 1,000 milliJoules so you need a big cap, like 20,000uf. Of course a bigger cap is going to take more time to charge. Since the dwell of a points system is so long, that may not leave enough time between coil charges to charge the cap. Re-phasing only makes this worse since you are cutting the cap charge time for the right cylinder in half.
The HEI is better suited to running without a battery. Dwell at 1200rpm is around 8% and it draws less than 200 milliJoules. That means that it will run with a smaller cap, like 1,000uf. Plus there is more than enough time to charge a larger cap, even when re-phased.