whether you consider it serious or not is your decission, but to fit the 68 pan under you need to move the rear body mounts where the shock towers bolt to the body.
on the 68 pan the towers sit higher so you can either cut and shorten the current one, move it up or poss (has anyone out there done this?) fit the later body mount onto the oval shell.
apart from that there might be a little miss alignment in the sill/pan bolt holes but that is nothing serious.
henry.