Powerglide is the Chev designation for the GM 2-speed transmission and is essentially the same transmission as the ST300 - other than the bellhousing bolt pattern.

A better more modern choice of transmission would be the 200-4R which features slightly wider gear ratios than the TH350 and adds an overdrive fourth gear for economy.

It is the same length as the ST300 and uses the same driveshaft. The crossmember from a TH400 will fit with minor mods for the trans mount. The 200-4R uses a cable to tell the transmission how much throttle you are using so it can shift at the right speed and with the right firmness, so you'll need to make a bracket to hold the cable. It also has a lock-up torque convertor for still better fuel economy and there are kits to make this functional in your car.

The better 200-4Rs are in cars produced after '86. The one that I have was in a 1987 Pontiac Grand Prix with a 4 bbl V-8 (Chev with the dual bolt pattern so it'll fit the OHC). It'll handle anything the OHC six can dish out with no upgrades needed. This is my choice for a cheap modern automatic to put behind the OHC. If you find a good used one you can use it as is and save yourself the cost of a rebuild/upgrade.

One thing to consider, the overdrive gear wouldn't be usable with really tall gears (2.56:1 for example) but you could use the money saved by buying a good used one like I did to upgrade your rear gear ratio to 3.55 - 3.90:1.

Here're the overall gear ratios with the 200-4R: first is 10.67:1 (2.74 x 3.90), second is 6.12 (1.57 x 3.90), third is 3.90 (1.00 x 3.90), and fourth is 2.61 (0.67 x 3.90).

The 200-4R with 3.90 gears would give the same off the line acceleration as a TH350 with around 4.30 gears but still have the same fuel economy as a TH350 with 2.56 gears! The best of both worlds, performance with economy. That's why I'll be using the 200-4R with 4.11 gears with my 250 OHC in my '69.