A short bed camper on a long bed truck is not an issue as long as the truck is capable of carrying the weight. COG is not an issue, as long as the truck is capable of carrying the weight. At most it will be ~10" behind the rear axle which is well within the manufacturer's COG "zone."
If the camper doesn't have "saddle bags" (rear external storage compartments) it can be slid right to the front of the long bed. With storage compartments, some sort of 18" spacer is needed to keep the camper from sliding forward and wrecking your bed. This spacer can be as simple or sophisticated as your imagination allows, and it makes a handy external semi-covered storage area for whatever you want to put there.
Long bed camper in a short bed truck... It's been done but it's less than ideal. Long bed campers are typically designed to have the first 8' of floor supported, and may not hold up well to having 18" of floor hanging out with no support. The tailgate doesn't count as support, as all that's holding it up are those two spindly cables. Tailgates are just not designed to have 1000lbs of weight bouncing up and down on them for thousands of miles.
Again, people have done it, and have done it with success, but it is less than ideal. You're much better off putting a short bed camper in a long bed truck than vice versa.
