I do not recommend adding additives to fully formulated engine oils. OEM-style flat-tappet valve trains only require around 1200 ppm of phosphorus (the anti-wear component of ZDDP). You're much better off getting an oil that already has the right amount of phosphorus. The latest API CK-4 heavy duty engine oils have enough ZDDP as well as detergents to keep your engine clean.

STP is a poor way of adding ZDDP as it has less than 800 ppm of phosphorus and gooey polymers. See HAMB - STP Discussion.

See the following articles for more information:
Selection of the Right Motor Oil for the Corvair and other Engines
Engine Wear
Heavy Duty Engine Oils

1965 Barracuda, 225 slant six
Rods & Relics - Fort Erie, ON