This is a look at the proper location for drilling and tapping for the boost gauge on a 7.3L if the "T" fitting installation isn't desired.   Thread size is 1/8"NPT.  Requires a size "Q"or "R" drill bit (0.332"-0.339"). Refer to the tap for the exact size (most have it printed on their shank).   This is just an optional method for those who don't wish to use the "T" fitting kit shown in the previous image.