There were a few Powerglides used behind Pontiac engines, back in the very early '70's. I've only seen 2. One was in a Bird, the other in a Lemans. I bought the Bird. These Glides had dual pattern cases. Sometimes only the BOP pattern holes were drilled. These dual pattern Glides are extremely rare today.
But, if you really wanna use a Powerglide, behind a Pontiac engine, a simple adapter plate is all that's needed, to bolt the Chevy trans up to the Pontiac block. Then, rebuilt Glides are available from LOTS of sources. Or, you can buy one with a JW Ultra Bell, which will bolt up, without an adapter plate.