Match the regulator to the output of the alternator. 37A alternators use 42A reg, 55 and 63A alternators use 63A reg. A/C cars usually use the 55 or 63A. It's stamped on the alternator case along with the part number. If the alternator has been rebuilt, it may have output higher than the part number would seem to indicate.

'69 changed to internally regulated alternator to avoid the charging issue you are experiencing. The original regulators were electromechanical, with coils and contact points. Most service replacements are transistorized. You can sometimes tell which you have by watching the dome/courtesy/instrument lamps just after starting the car. If you see a flicker, like fluorescent lamps or a computer monitor, you have a solid state regulator. If the lamps merely dim a bit at idle compared to revved up, it's likely electromechanical.