Lots... There were a ton of changes 68 -> 69. I think the roof is the only piece of sheet metal they have in common.
Look for a '67 and leave the quarter windows out. Thats as close as you will get. If its an RS you will be really close, except for the tail lights. If its not the front parking lights will be round instead of square. (Interiors are almost identical 67 to 68 as well)