I have a 1955 Cent 2dr ht. It currently has a 1956 dynaflow transmission in it that I installed. The 1956 Dynaflow is 1" longer than the 1955Spec&Cent
You can install the 56 dyna in a 1955 Super or Roadmaster much easier becuz those transmissions were equiped with a longer tailshaft. I took care of the extra length proble by studying the different torque balls used in that era. I used the 1948-52 torque ball which is shorter than the 55 Sp/Cent. I noticed my 55trans cross member was 'offset', I also had a 1955
Standard trans crossmeber which is straight, but still needed to have the bolt holes slightly modified. When I brought the torque tube and put the splined end in the U-joint it stopped with an approx 3/8" gap. Since ai had installed a shorter toque ball the splined front of the driveshaft was too long and ran into the center of U-joint yoke, so I pulled the driveshaft back out and shortened a bit more than 3/8" and chamfered the end as original. The trans fluid cooler on 55s is mounted at the back of trans. The 56 has cooling lines running to the radiator. I bought a remote cooler and installed in front of radiator. The reason the 56 is longer is Buick added another stator and needed more room in the bellhousing. It performs better at takeoff cuz they changed the ratio affecting acceleration. The fine splines in the U-joint evidently had quite a few failures, some being twisted like a piece of licorice, probably from more stress on the joint from increased HP and the torque multiplication with the changes in the convertor. Just a SWAG((scientific wild ass guess)! Good luck! I consider it an upgrade until it proves itself otherwise.