Soffit vent install problem?

Yes. Use the round face vents and the biggest you can get to fit the board. Space these about 2ft apart and start at the center. As for the gable vents not doing their job; remove enough material or (slats) from the gable on the east or north side and frame this in from the attic to accept a fan driven vent, electrically controlled with a thermostat. I'm uming you have electric in the attic. When the fan kicks in it will exhaust the hot air to the outside. I just installed one of these myself, setting it at 90d. This is the best way to do it. The regular venting just doesn't do it because they are dependent on the wind. Good luck!