If I did the math Right.
Gray 15 tooth Drive Gear on your output shaft should be a 34 tooth Lt Green Driven Gear
Red 17 tooth Drive Gear on your output shaft should be a 39 tooth Brown Driven Gear
Blue/Green 18 tooth Drive Gear on your output shaft should be a 41 tooth Yellow Driven Gear.
I'm going to take a guess and and say the Gray 15 tooth is more common. Now you having a Th400 in a half ton with a tall gear, makes yours possible to be uncommon, and why I gave you more than one option. You;ll need to stick a flashligh in the speedo hole and look at the color and/or count the teeth on your Drive gear to know which driven gear you need.