Please note on TH400 transmissions, their are two seal diameters, this is best measured by measuring the O.D. of the seal bore in the extention housing, it will be either 2 1/2 or 2 11/16 also TH400 should have a 31 spine output shaft if you count 27 splines your transmission is a 375 please mention this when ordering your shaft.

NOTE: Driveshaft measurements must be made at vehicle ride hight (Axle(s) must be supported and not hanging down) and at complete vehicle curb weight (Engine installed, body on frame etc.) this will help provide us correct measurements.