I'm not familiar with '68s, but this shows the spacing of the '60-'66 armrest bolt/hole locations.
http://rmcavoy.freeshell.org/arm_rests.html
You should be able to compare that to the mounting holes in your door and tell whether or not they are gonna fit.
The armrests on a '60-'66 are attached with 2 screws each, 1/4"-20 thread, that go through the armrest, through the door panels, and into a pair of square nuts welded to the door itself. Some doors, like my '66, are missing these nuts, but they can easily be replaced with 1/4" cage nuts off ebay (same nuts hold the door panel on). DON'T just sheet metal screw the armrests to the door panel unless you want it to bend the hell out of the door panel.
Again, I do not know what of this applies to '68s, but that is how it works on a '66. The guys in the '67-'72 forum could likely tell you more than I can about your particular model.