For '68 cars, the colored "rubber" knob is held onto the metal crank with a special "push" type fastener.... almost like a metal version of a plastic xmas tree used on a late model car to hold a door panel on.
When you buy a new knob for your '68, it'll come with this special fastener. To install the fastener, lay the knob on the workbench over a towel..... place the crank over the knob..... place the fastener into the backside of the crank and into the knob..... with a hammer, gently tap the fastener into the knob.
To prevent the knobs crank from hitting your knee when the window is rolled all the way up/down.... remove the screw holding the crank onto the regulator and reposition the crank finding that "sweet spot" where the knob doesn't hit your knee in either position.
65, 66 and 67 cranks will work on your regulator..... they won't be "correct" for the car, but they'll screw on okay.