Is this the correct cap for 68?
That is almost the correct cap. That is a service replacement cap that has the same form but the text on the originals was different. Originals stated to use automatic trans fluid Type A. 67s also used that cap. Only 66s used the domed cap.
Here is a photo that includes an original 67/68 cap. The photo has enough resolution to read the text on the cap. "USE AUTOMATIC TRANS FLUID TYPE A"