Ask and you shall recieve! (Flip Top Gas cap Adapter)

Hmmmm.... As for modifying the adapter... have at it man. That would be my go to answer if it were me.
As for the different depths.... It doesn't really appear that its any shallower than most. Granted we are all trying to decipher this by a picture, but it doesn't look the difference that yours seems to be sticking out.

As for you having one of 3 74/75s.... obviously not, but you do seem to be among the very few that have come across this problem.

Looking at this, and putting a little thought into it, I cant see any obvious answer for you other than to modify the adapter by removing a little material from the backside.

HOWEVER!!! Be aware, that if you remove it from the back surface, that the original filler neck surface will be the next surface to cause an interference.
In that case it wouldnt be as easy to remove material very nicely with general hand tools.
Ill see if I can post up pictures of mine when I first installed it. We can try to use it as a comparison.

As for the end result..... Because you are among the very few that has had any issue, if you have NOT modified the adapter at all, and if you are unhappy with having to modify the adapter to fit your car. If you will send it back I will refund you your cash and shipping charges.
I would like to "right" the situation and allow you to be content with the end solution whatever it may be. :)
Donnie