Late to this topic, but including a photo of the correct '68 500 dashpot and bracket. Notice the bellows does not have an external coil spring. Ford complicates this part by continuing the part # (without changing the engineering revision suffix) when servicing the "long arm"/plunger version that was used on CJ and B9, etc.
Now, the question of 350 application and tranny---I have not studied.
Don