Once again, the "Theorists" are out in full force. A PG is lighter, requires less maintenance, requires less HP to run it, is just as strong as a TH400, way stronger than a similarly built C4. In "theory" the 'glide would need more gear. But in reality it does not, and I have at LEAST 4 or 5 firsthand experiences with this. All 3 of mine, a '68 coupe with a mild 408, a '72 Maverick with a 408, '85 Mustang coupe with a 302" (yeah, with a PG....remember, "it won't work"), '57 Chevy with a 355, etc.
If it needed more gear to get it rolling, you'd think that keeping your current gearing would cause the 60' to suffer. In EVERY case where family, friends, and myself where they chose to switch from TH350, TH400, C4, C6 or TF727 to a powerglide, whether the 'glide be a 1.82, 1.76, or 1.80, the 60' time IMPROVED. BUT on that note, the converter has everything to do with it. You put the wrong converter in front of it, and you'll be rewarded with lackluster performance. A good example is a guy I know as "MG". '88 Mustang, BBF, makes about 600hp and probably close to 600 lb-ft. Was a C4, ran 1.41 60', and 6.40 at 108 in the 1/8 with the C4. He asked me to build a glide so I did. Problem was that he said he already had a converter, 4800 stall from an old project that was a TH350. No problem right. I advised him to send the converter off and have it rebuilt for the 'glide and the car. Unfortunately he did not, and paid the price. After the install it went 1.46 6.45 at high 109mph, might as well say 110. Stall speed is ONLY part of the equation. He got mad at me and kept saying something was wrong with the transmission-and asked me to pull it down and inspect, so I did-twice. The 2nd time I called him & told him he needed to buy a few gaskets and he got mad again, said something was wrong with the transmission, blah blah. I knew it was the converter from day one. I lent him my 8" CCX 5200 converter, he put it together and went 1.36-6.31 at 110. And even then my converter didn't really match his engine, probably a little too aggressive at the hit since mine's a small block making a little less torque.
Your converter will also help on the big end too if it retains the sprag. I had mine built spragless since all I do is bracket racing, and it works for it's intended purpose. It's slightly more consistent than with the sprag. 1/8 mile only. If you're running 1/4 mile, keep the sprag, it'll help on the back half of the track.
So then why do you need more gear? high gear is 1.00:1 on almost all 3 speed transmissions. And it's also 1.00:1 on a powerglide.
Now I bleed Ford blue just like the rest of ya, but the day I went to the track with a powerglide, I won...and was SOLD. Have not looked back. Street car? They're (PG) not all that great on the streets. Streetable-yes-depends on how much you can put up with.
The TH400 has a sprag clutch inside it....not that it's known to fail, but they sometimes do. I've not seen one personally explode, however, I have seen a C4 and a TON of torqueflite 727's explode due to failed sprags, and it's not pretty. The TF727 was THE reason a transmission shield was invented. PG does not have a sprag. The arguement at one time was that "when it burns the band up, you'll wish you'd stuck with a 3 speed"....a PG band doesn't burn up unless it slips. It doesn't slip if the servo is holding like it should, and where some get into trouble is trying to run a stock single ring servo, they leak, and burn up the band and high clutch pack & drum. Put an aftermarket servo in-problem solved.
Every single part can be bought aftermarket to build a PG, including the case and every nut and bolt and spring. This allows you, the racer, to build it however you need it. If you want one to handle 3000 HP, fine-it can be done. Easily.
The 400 is also heavy. And uses up some HP to run it. But it's very strong. I compare them to the big C6, power hungry but hard to hurt them. TH400 properly built is also a little on the expensive side once you figure in the adapter bellhousing, internal parts, valve body, converter, etc. And it's pretty big, similar to a C6. Lot of guys use them, successfully. After running C4's and C6's for 20 some odd years, I made the switch back in the early 2000's to a 'glide and never looked back.
Powerglide is also REALLY easy to build, which if you're handy with a few tools, saves you some money.