I'm not 100% sure what your question is pretaining to...
If you are asking if you could remove the 71-72 front suspension parts (spindles, disc brakes,...) and install them on your '68 and then have disc brakes... then yes you can do that.
If you are asking if you can swap the arms and still keep the drop you once had and run drums... I don't believe you can as the spindles are setup differently.
You can always contact
ECE,
CPP, or a
board supporting vendor and pick up a disc brake conversion kit from one of them.