The driveline - center of the crankshaft bolt to the center of the pinion shaft - should be straight, as far as side to side, and parallel to the chassis centerline.

The A Body stuff I've dealt with was like an 1 1/4". And I believe that number holds pretty close for the other chassis. Tolerances being what they were, that 1 1/4" could be +/- 1/8 or more. Drop stringlines and get it as close as you can.

If you're using all OE mounting, not much you can do with it. Maybe fudge the trans one way or the other.

If the results don't match the theory, change the theory.