rear bumper gap

Hi guys I have a 71 swinger and just put the rear bumper back on and saw there is a 5/8- 3/4 inch gap between the bumper and the body-extentions. Is this just the nature of the beast .I did no work on that area so that's how it came off. Has anyone tried shortening the bumper mounts, or do you just run into problems you can't see Jim