As has been stated, the TH 350 didn't appear until the '69 model year. The TH 250 is a decendent of the 350, using an externally adjustable band in place of an intermediate clutch pack (for 2nd gear). The 250 appeared about 1974.
The Super Turbine 300 is a two speed A/T based loosely on the TH400 and used in BOP applications until about the '69 model year. Having overhauled many of both forty some years ago I always felt the 300 was a better trans than Chevy's Powerglide.
Assuming this nice Firebird retains it's original trans, and it is a two speed, it should be an ST300. But then, many two speeds were upgraded to three speeds back then.
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