Yea, the -6 plans showed the general location of the NACA vent...no actual dimensions.
Using the best possible TLAR engineering (That Looks About Right), here's where mine are located:
View attachment 53472
Location dimensions (actually measured off the plane this morning) are:
1. Horizontal: cowl side skin (R side in photo) to forward (small) end of the NACA duct: 2 1/4 inches
2. Vertical: top forward skin edge to NACA duct center line: 2 3/4 inches
The vertical location is the more critical of the two. If the duct is mounted too low, it can interfere with the rudder pedal mounting blocks.