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The angle of the trans is relative to the angle of your differential. pull the driveshaft and put an angle finder on the face of the input pinion flange ( or whatever passes for a flange). The corresponding flange on the back of the gearbox needs to be the same but opposite.

The goal is to have the center line of the gearbox output shaft and the center line of the pinion parallel but not in line ( concentric). In line will eat u-joints and non-parallel will cause vibration.