I bought some standard belts for a 71 Duster 225 a while back on Rock Auto under the Dayco part # 15570.

I painted the engine up and threw away the belt that was on it a while back without making note of what the part # was on the belt. I go and grab one of the new 15570 belts and it seems just a little too short. I feel like I used to be able to swing the alternator over to where it was touching or very close to touching the valve cover but now the furthest I can get it is an inch and a half or so away because the bottom of the alternator hits the alternator bracket beneath.

I have an old 15575 belt that's slightly longer that I can get on the engine to get it running but I have to pull the alternator much closer to the inner fender than I ever remember doing in the past. I know this is a stupid question and there's probably a simple solution but I can't seem to figure out why the standard belt no longer fits.

Same experiences here.

I tried various 15550, 15555, 15560, 15565, 15570, & 15575 which have lengths ranging from 55 to 58 inches & found manufacturing variations in the belts of the same model and same limitations of hitting valve cover versus inner fender well in my 65 Dart. Generally I go with the one that gets the belt "tight" closest to the valve cover since over time it needs tightening and the adjustment move the alternator toward the inner fender well. Right now the 15565* belt is decent on my 225.....at least when that particular belt was manufactured.

Some really old places will measure your belt & let you what it's real dimensions are so you can incrementally change the size (theoretically).

Bottom line: I developed a procedure to deal with the quality variations inherent in a belt size manufactured on a particular day.

* NAPA similar number is 257565

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

Keep in mind typical auto V-belts will stretch and require re-tensioning after so many miles. After break in a belt that lets the alternator come close to the valve cover should be "just right" when re-tensioned.