Tom is right. This would be a tough one or almost impossible to convert to a two speed rear end.

The torque tube was a driveshaft totally enclosed in a tube. The front universal ran in oil and there was no rear universal. The motion of the shaft was dealt with by a large sealed ball joint type assembly on the transmission/tube junction.

The downside of this setup was it increased unsprung weight and was possibly costlier to produce. A known difficulty was the rear seal in the setup failing in the transmission junction area. This allowed thinner transmision oil from the dynaflow transmissions to flow down the shaft into the differential.

This setup was a logical way to engineer in coils springs for the rear end. I am not sure if buick was the first to employ them back then or not. Maybe cadillac as they were Toms passions for a time so he might know.

Back then Cadillac built quite a car by the way. After decades of having good engines they put what I consider junk under the hoods starting in 1957. Junk in my opinion that is compared to what had been.

I think I got about twenty miles per gallon around town with my 1954 model cadillac. The only cadillac I ever owned. The new engine starting in 57 probably did half as well milage wise. There was no lack of ;power in the 54 either. Cadillac slogan the standard of the world was ultimatly thrown away by the corporation eventually.

Like the mercedes constant velocity joints the buicks single universal lasted almost forever with the constant oil lubrication.Buick continued the torque tube set up into the 50s.The same torque tube set up is on the standard transmissions.

My 39 buick is converted to a standard type rear end and driveshaft. The buick torque tube both would have been impossible to hook up to the 350 auto transmission and even if possible probably would not have survived the current engines torque. This car is heavy and goes like a bullet if you get into it.

The most severe long hill in the maritimes makes most new cars work pretty hard to maintain speed on. Most in fact cannot hold the desired speed. The 39 buick resto rod does not even seem to hardly notice it as I pass all of them. This hill would force a 240d standard to stay in second gear. You cannot get a real run at that long hill. Even if you could you would still land up in the second gear part way up I think.