They used 400's in the Firebirds up until 1979 when the 400 was discontinued. The 350 will work as long as it is maintained with regualar fluid changes, a big magnet in the pan, an auxillary tranny cooler and a shift kit. The shift kit with its harder than stock shifts, decreases the slippage of the clutches and therby stops them from wearing as much.
I have the stock th-350 that came with the original 301 in my 1980 firebird and I have well over 200,000Km on it. I have grenaded 2 well modified 400's and am currently running a hot 455 and the trans still works perfectly. Maybe I'm just lucky, but I think its becusae of the way I take care of the transmission.
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