1. I seriously doubt that your air compressor can accurately regulate down to 6 PSI.
2. I doubt that your Harbor Freight leakdown tester has a schrader valve in it.
3. Are you planning to just let the compressor gleefully blow air into the motor as fast as it is leaking out? If so, how are you going to know if you have a leak? Your leakdown tester certainly isn't going to tell you.
4. Although I haven't tried the mentioned scenario, I seriously doubt that the reeds seal air tight at the low velocity of air flow that is going into a motor during a leakdown test. In other words, I don't see pressurizing through the spark plug hole as being a problem, if done correctly. However, that's not how it's normally done.