I have been told that there were no production number breakdowns as far as trans options but the Hydramatic was first in Chevy in 54. GMC got it in 53. In 55 1st series they went to open driveline. Parts are pretty rare and if it is anything that can be saved, You really ought to jump on it. I installed HydraMatic driveline in my 54 and that is the way it should have always been in my opinion. Chev linkage is different from GMC. Many other items are different on HydraMatic trucks designed around trans access and removal from bottom of truck as well as solenoid start. I f you choose not to get it, Let us know. I am sure someone else would be interested in it.