Please pardon my ignorance but I haven't done any drag racing for 30+ years.
A PowerGlide is two-speed, right? So, how would this be better in the quarter mile than a C6 or C4 with three speeds. Are these BBFs so powerful that two speeds are an advantage?
Just my opinion.
A powerglide is lighter than a C-6 with much lighter internal rotating parts.
It's stronger than a C-4. Besides the strength, the powerglide has a much more surface area on the internal clutch plates.
Powerglide has more aftermarket support. Every single piece inside and out is available from the aftermarket. If you wish, you can build a complete unit without a single GM part number. You can also take a stock unit, add a few hundred dollars in aftermarket parts and it's good for a a lot of power. The sky is the limit on how bulletproof you want to build it.
Simple, with less parts less to break. It takes longer to pull it and install than it does to freshen it up.
The single shift on a pass makes it easier to be consistent. Also, the higher gear ratio of first gear vs a 3 speeds first gear calms the car down a little on launch and can help when track conditions are bad.
Torque converter selection IS crucial though. With the correct converter I've never seen one that was slower than a 3 speed, most were quicker, with a few showing no change it the 3 speed combo was optimized.