odd alternator pulleys

[geeves]

About 4 years ago I was in picapart Wellington and saw what looked like a near new alternator in a Mu that was being wrecked. Didnt need it then but we all know how important it is to have a spare so $80 later it was in my pats box. A couple of months ago the one in my truck failed so a slow swap was done. When its been at least 5 years since it was last touched and everything covered in mud it was not a pleasant experience. Every bolt had to be undone all the way with a spanner. Lots of oil reassembling as well so next time should be a treat.
Anyway I noticed the new alternator was quite a bit bigger than the old one and the voltmeter suggests the output is a lot more as well. Unfortunately there was no label on it so no numbers.
Now for why this is on the faults page. I couldnt stop the fan belt slipping even under far more tension than normal. Took the belts off today and noticed the pulley is different to the other pulleys the belt runs on. It is a V but has a flat bottom and it looks like the belt has been riding on the bottom of the pulley. Belt is a Gates 15A1015 which isnt new but no cracking and it still measures 15mm wide.
Will fitting a wider belt help or do I need to change the pulley? Can I change the pulley in place?