Not sure what transmission you have, just a quick look in the parts book, the easiest comparison was for a 68 with C-6 and a 8.15 or F-70 tire. It showed for the 2.75 gear should have a driven gear with 16 teeth, and for a 3.50 gear (couldn't find 3.55 with same tires ect. but it's only 1/2 a percent) showed a driven gear with 20 teeth part#C1DD-17271-A. Most all of the gears interchange even though they have different styles (short or long teeth) just make sure the angle of the teeth is cut the same way. I check earlier trucks in salvage yards they sometimes have lower gears. More driven gear teeth slow down the speedometer. The highest count in my book is 21, after that you need to change drive gear output shaft or add a reducer box. There might be more but my book only goes to 72.