On many alts the pulley is set to sit over the key and the smooth part of the keyed shaft is not wider than the pulley so when you shim it out you are losing some shaft & keycontact. A few mm's is probably not a big deal on a 90A alt but could be on a larger one. On some alts this can be done but not on all unless we're talking very small distances. I actually prefer to mil the alt foot if it is say a 2" foot rather than the bracket as many of these same cases come in a 1" and 2" foot design. If the pulley needs to go back then shimming won't work unless the back of the pulley can be machined away some. I have also done that before. Ron's Auto Electric on Route 17 in Windsor can machine pulleys for you, he's done a number of the cast/machined pulleys for me for very short money. Many ways to attack the alignment..