Seems to be the right T 19 but my Buick no.is 4.337-1168737.This is if you have an axel ratio 3.36.
Driving gear no.is as yours 4.343-1171028.This no.is only for 1956 DF.
But in my opinion the problem you have is in your speedometer,I think you have to adjust the speedometer.
Leif in Sweden.