What is the differance between a disc spindle and a non disc spindle. This is on a 67.

This is a disc-spindle (69-72, 67-68 may be slightly different)
http://www.chevelles.com/showroom/data/516/medium/Disc_spindle_dim.jpg

On a drum spindle, that upper bolt boss sticks out farther (measurement would be smaller) by I think about 3/8"

on a drum spindle, that to boss sticks out a little bit farther than the lower steering arm bolts- not the spot where the seal rides like in the pic Derek posted- by about 1/4". if i remember right, you take off about .6" to make a drum spindle into a disc spindle.
but if you are doing a home made C4 or C5 Vette setup, the easiest way to go would be to cut the top boss down even with the lower bolts.