Fender extension question

I have a 70 340 swinger, the fender extensions (rear) that came with the car had broken studs and were in bad shape. I have a pair off a 71 but they fit like crap the old ones line up better. Has anybody else seen this? Are they different or is it hit and miss. The quarters have been replaced both n.o.s. not repos. They don't even line up good with the trunk lid.