Maybe one way to know for sure is to pull your door panel and make sure the speaker will fit where you want it to go. This will give you a better view of where the speaker will fit and where it won't. This way you'll know the magnet will clear and obstacles. Hope the best.
I installed 6" speakers in my kick panels 65nova 400 coupe and had plenty of room. Not sure on your 68 kick panels but,I'm sure you have plenty of room in your doors. Test the depth and good luck. Cheers, Pat