Welcome to the Stovebolt site. There is another site you need to check out also.
http://www.oldgmctrucks.com/ Check out the site and sign up for the forums. There are several people knowledgable on the hydramatics and I'm sure someone has a source for pistons.
I'm not trying to run you off here. We have a great bunch of guys and gals here you'll enjoy getting to know.
