If my memory serves me correctly, the alternator mounting on these older 240 series has a bolt on each end of the mount that has to be loosened in order to move the alternator, and tighten the belt.

I remember when I tightened the belts on the older 240's that I've owned, I used to loosen both bolts and then i would use a pry bar to lever the rear of the alternator out . I did this without disturbing or moving the belt adjustment (tension) bolt.
After I had the rear of the alternator levered out against the belts, I would then snug the rear bolt.
At this point, the appearance of the alternator would look to be out of alignment with the belts, and the front of the alternator would appear to need adjustment outward against the belts.
I would then use the adjustment bolt to pull the alternator into alignment with and adjust the tension of the belts.
(With the back end fixed by the rear bolt, the front of the alternator would be pulled outward and the belts would move into alignment)
I would then tighten the front bolt, and then the rear.
I basically was using the adjustment bolt as a pivot point to align the pulley with the belts.

The one part that took a little practice was the prying the alternator out against the belts.....learning how much tension to put on the belts so that when the alternator was finally pulled into alignment, the belt tension would be right.

I hope that made sense, and that you can understand it.

steve