new belts on diesel motor squeaking

I had my belts replaced on my Cummin's engine. Fan came apart so I too thought PM of belts and hoses was a good idea at that time. I can't believe the problem finding the right belts for my engine. Cummin's had a number, didn't fit at all...Freightliner had numbers and only one of the two fit. The other belt was finally found by trial and error.
No belt should squeal on installation. Only two reasons, a bad bearing on the altenator or idler pulley(s) or AC or the wrong belt used. Actually a third, the belt was not adjusted properly..