Several Questions

[raychevy]

I own a 76 chevy made at the end of 76, so I guess its kinda a 77. The truck has 350 engine, with the automatic transmission. I believe the tranny is the Turbo 350, because the shift indicator has L1 and L2. When I bought the truck it had been sitting for several months. Sometimes it would not engage drive until I taped the gas, then it would without a problem. After driving it for a few weeks the engaging problem went away. I know the fluid in the tranny is light brown and nasty, but it still shifts fine. I want to change the fluid and filter to freshen it all up. I have heard people say that changing the fluid on the old trannies sometimes makes them fail. Has anyone seen that ? To change the filter dont I just remove the tranny pan, and put a new filter in there, and new gasket ? Then how much fluid do I pour back in ? I am looking at changing the entire cooling system this spring. Should I wait, and do all this at the same time ? I have never dont this, and need all the help I can get.
Thanks in advance
Ray