The cheap "A" - section belts constantly stretch and need adjustment.

They were replaced with as close to "B" belts as practical. Picture, see how the belt 'rides' higher in the sheave (pulley belt groove),
full contact with the belt groove and the top edge doesnt wear on the side. That prevents damage to the belt top cover.
These belts have been on this engine about 20 YEARS and I havent adjusted them in at least 10 years and they are still tight.
This truck doesnt see many miles, but still MUCH better than the skinny belts.

If/when replacing skinny belts, sand the sheave grooves clean and shiney to prevent damage to the new belts, or, degreaser and Muriatic acid to dissolve the rust.

Gates numbers:

7420 /11A1065 PS/AC
7448 / 10A1135 alternator/water pump belt
I cant see the number on the third belt, just measure or Xreference with belt of the same series