Think I figured it out....
I think it relates to the armrest mounts being in a different place for the '68. Found a drawing that shows 67 using similar clips, but obviously in a different location....
And the fact I want to convert it to a '68 deluxe now indicates I may need to source a '68 shell.... bummer... Its a nice straight/clean shell.. may have to look at some metalwork from my shop... may be cheaper than sourcing a new door...