I seem to recall the question had come up on the Forum at one time whether Buick issued a factory kit to convert a Flight Pitch or Triple Turbine equipped car to a Twin Turbine transmission. This should answer the question.
This is from my 1940-63 Chassis Parts Book.

Gr 4.005 All 1958 Part # 1396090 "Kit,3T Trans To 2T Conversion Parts".

Gr. 4.005 All 1959 Part # 1396091 "Kit, 3T Trans To 2T Conversion Parts".

Unfortunately there is no listing as to what the kit contained. The 58 kit was priced at $27.50 while the 59 kit was priced at $20.00. My guess would be a different stator and throttle linkage, shift dial indicator, and transmission mount hardware recalling my past experience doing one.

There is nothing like driving a Flight Pitch equipped car. Have yours rebuilt instead of converted if at all possible. Jim Hughes, BCA member here in Perrysburg does a superb job and is very knowledgeable on the trouble prone items.
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Dave Rex BCA 1649 1939 Special 4 door 1965 LeSabre Conv 1997 Riviera 2007 Pacifica