The first thing I would suggest is to find the templates for these cases that are scattered around the forum, print a couple, and see which fit you prefer, as there are a couple variations on the dimensions and some are a tighter fit than others.
The "right" way is probably to rebuild the cover from cleaned scans of the original retail covers using your preferred template, but that's a lot of work. If you've tried it, you'll know you can't just resize the whole image to match the CGC dimensions. The absolute easiest way to make everything fit would be to split a UGC cover into three separate images: front, back, and spine. You can then scale each section to fit the corresponding of the CGC template. You'll end up with a bit of aspect ratio distortion, especially on the spine, but nothing too terrible.