Thanks! Why don't more people know about this! Better question, why didn't Chrysler just do this in the first place?
Back in the days of v-belts, there was an orientation that "essential units" would get a dedicated belt to run them. In this case, the water pump, which makes sense. If all of the other belts fail, the water pump would still be operational. The original Z/28s used a dedicated belt for the water pump sans tensionser, but used special deep groove pulleys to accomplish that. Plus you also have to remember that v-belts can stretch with age and use. There are some recent GM a/c compressor belts which are untensioned, with ONE belt that runs them. Talk about a pain to change the belt on, if needed! Obviously, v-belt quality control is better, decades later, than "back then".
Remember, too, that with the dedicated-length belt/pulley arrangement, if the belt goes and no exact replacement is available, not a good position to be in. Especially in a world with land-line phones and no "pay phone change" in your pocket . . . at 10pm at night . . . away from the larger city. Much better to "have options" of making something different possibly fit and work well enough to get you back to civilization or your normal repair shop!
Before getting all "excited" and asking questions (in the current time) as to why they designed something the way they did (in the "model-year" time, minus 3 years), you have to remember the way things were done back then and WHY they were done that way. It's those side issues, "two levels below the surface" that usually explain things better than what's seen on the surface, in current times.
Another side issue would be Chrysler's huge law enforcement vehicle business. In that area, having a vehicle that can go 24hrs/day is important, no matter what. So finding a rouge belt that would work would be more important, ultimately, than having to have the EXACT item itself. That business was extremely important to Chrysler, even if it might cost them a few dollars more for the belt/idler arrangement for the water pump belt. Further solidifying the "one belt/function" general orientation for essential engine accessory drives. In this case, the water pump would be an accessory, just as the alternator or power steering pump would, just that the alternator or power steering pump would not be essential to engine operation and its longevity.
As time progresses and later re-designs happen, other options did appear as parts quality did improve.
Just some thoughts,
CBODY67