Pinion Nut Size
I assume you are referring to the thread size on your pinion gear. Any "9 Bolt" gearsets we have worked on or sold have a thread size of 0.750" and a thread pitch of 16tpi(threads per inch). The original nuts from the 1985 to 1991 era have a flange behind the hex body, like a Ford nut. However, we have been unable to obtain that style for some time now and have been using the Dana 44/60/70 nut along with the washer Dana specifies to use with this nut. Any GM, Ford, or Chrysler dealers Parts Dept. should be able to supply you with this nut and washer. If not, we have them.
Have a machinist friend check the thread dia and pitch on your pinion before purchasing.