"Powerglide" trannies were a Chevy 2=speed unit, with a Chevy bellhousing pattern.
"Super Turbine 300" trannies are the 2-speeds for Buick-Olds-Pontiac, with that bellhousing.
I understand wanting to install the TH400, but a TH350 fits where the ST300 was, using the same crossmember and driveshaft. Only difference is the kickdown is cabled instead of electric. I've done a few and you won't have a shifter detent for the extra gear, but you can live without that, or swap in a three-speed shifter of some sort.
But for the TH400, yes they were available, so you can find a crossmember and driveshaft that already works.
Glenn Uettwiller
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http://firstgenfirebird.org/show/closeup.mv?CarID=209
http://firstgenfirebird.org/show/closeup.mv?CarID=210
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