I could not find my original th400 tranny tube. I searched for a long time before calling "fabulouse firebirds" and getting a used one.
One I got I figured out what it looked like I found out I did have the orginal so now I have one for my second 68. Yeah!
I spent a whole day messing with it.
My sure fire way of having a leak free fit is to always change the rubber O-ring to start. Then fit the tube in place. Make sure the metal strap that secures the tube is perfectly lined up with the tranny bolt hole so you don't have to [censored] the the tube in anyway to make it fit.