I made the same gear ratio change on my 77 this year. I'm running 255/60/15 tires and used a 31 tooth gear. The current speed function of the GPS unit I used to check accuracy shows the speedo is accurate (I have not taken the time to check it with a stop watch or radar gun). Looking at one of the online speedo gear charts I used, the 29 tooth gear you chose is right, or at least real close. Wish I could be of more help.
Is it reading fast or slow? How far is it off? Didn't I read something about each tooth changing the reading by 2-3 MPH?