Mounting the blower to the manifold (or any manifold) is a bit more complicated.
The length of the specific blower between the drive pulley and the discharge port may be a variable in that other nose pieces or extensions may exist. Simply aligning the pulley with your front drive may place the discharge in a bad place.
The blower's discharge port may not be the same size and/or shape as the manifold's intake window, and centering the blower is not always the best position for mixture distribution.
Many roots and screw-type blowers (especially older ones including original GMC) have rectangular ports, but the charge volume is heavily biased to a much smaller triangular area just behind the nose; the remaining opening does very little. Modern Eatons already have a triangle port, as do some high-dollar GMC clones (12-71 etc.).