First off, I need to know will the lower quarter panel off the 66 impala fit the 68 impala? I know the top arch is different but what about the lower section? I ask because I cannot find the full lower section of the 68 quarter anywhere, they seem to come in 2 pieces that cost double the amount of the full 66 quarter and that's just ridiculous.
these are the 68 sedan lower quarters and drop offs. not great but might work on your car. let me know if you want to work with these. frankly dont know if the distance from the rear wheel house to the tail pan is the same on your fastback and the four door sedan.