440 /super charger add on ?

What style idler pulley? If it's a swing arm, loosen it and move it back. You might not get it perfectly centered on the pulleys (doesn't have to be) but should be able to get it pretty close.

If, on the other hand, the belt is at opposite edges of each pulley then you may need to add or remove spacers under a pulley to bring them into better alignment. Within 1/4" alignment is okay.

The important thing is not to over-tighten the belt. They should actually be pretty loose when cold.