Hey hey !!
ohhhhh my ,,,, gas caps ..........
there are several ....... vented styles ...
it depends on the neck of course ...
is it an original 68 tank .....
IF so...... from the 68 cars I have had ...the most common cap is the deep sided style
almost an inch ..... is your tank a reproduction .. or original to your car 68 one ...
Scott
we can chat and send pics at
scott68gto@comcast.net
thanks