Chris - I did a check in various Chevy Master Parts Books. The 1962 Book (see screenshots below) probably has the most detailed listing of the various belts for your 3 pulley setup, depending on the original application.
I think the majority of the applications, including the 261 (depending on year), indicate the most common belt width is a 3/8" (0.3875"). That might put the 15000 series belts just a touch wide as the majority of those I scanned through ranged from 0.41" to 0.44" wide, which is more of a 7/16" (0.4375") wide belt.
I'd get a 3/8" wide belt, 42.5" O.L. like this
Duralast piece and see how it sits in the pulley grooves. As long as the bottom of the belt doesn't ride on the bottom of the pulley grooves, that will likely be your best fit and the one to stay put in the grooves when running.
