I just finished repairing a few 66 stereos, and after fixing up the circuits to make them play again (another discussion), I was left with the usual problem; trying to find a belt that wouldn't slip.
I figured out that a modern poly-rib belt is just several small belts stacked next to each other:
All it takes is to just carefully separate them into pieces by cutting along the grooves using several passes and medium pressure:
And vóila! six belts:
Then just slip it on. If you cut it straight, it will roll smooth and not slip. The belt will have a fabric backbone like the original, so it will not stretch.
The total cost of this project was $5.23 U.S., and yielded me 6 useable belts!
I know it's not OEM, but it works great, and cheap to boot.