Back when the cars were "newer", the ONE part number for a Chrysler Corp a/c dual-belt (factory or aftermarket replacement) was a MATCHED SET. NOT two belts of the same part number. With better manufacturing quality controls, that might not be the case in more recent times. You can match the production codes on the belts, if they are packaged individually.
Rock Auto is a good information source, at the very least. With respect to "belts", their listings might have the needed length/width in them. You can use their listings to see "what's out there" and then seek the same parts locally. The "string method" might give you a starting point, but not much else, with all due respect, as you'll probably get the measurement at the base of the pulley's grooves, rather than higher-up in the groove where the belt actually rides.
DO take your old belts for a match of width AND length. Probably might still be some ink stamped numbers with that information on them, which would help if you have to drive that particular car to the auto supply store. OR write it down before you go.
The "story" on the necessity of the "matched belts" for the Chrysler a/c and alternator use, as I recall, was that they had to be matched so that when they "slapped" at certain rpm levels and compressor load, one wouldn't knock the other one out of its groove, which could also take the radiator hose with it too. NOT a good situation! No a/c and no coolant. On the side of the road, in a time when all "phones" were landlines. Call those matched belts "insurance".
No matter how tight you can adjust the a/c belt, it'll always "slap" or "flex" as the engine runs with the a/c on. Some of the later cars with an idler pulley for those belts probably won't do it like the earlier ones did.
CBODY67
