Dave, the first 2 Holdens were styled after an early unreleased Chev prototype.
However the FE was badged as a Kapitan when being tested on the roads pre release. Maybe there is some Opel in the FE - though there sure is a lot of 1954 Chev there as well.
The FB was most certainly styled after the 1955 through 1957 Chev without a shadow of a doubt.
Some stories I have read about the EJ seem to suggest that it took a bit of styling from the Vauxhall?
Then the EH went back to the 1960 Chev for looks
HD was styled after American Buick and Oldsmobile.
Maybe the Opel and the Holden were all styled after Chevs - that's why they look alike.
Ken