Is it ever a good idea to just cut a branch back a few feet? I have a young plum tree whose upper branches are all pretty long, but don't branch out at all.
Perhaps this will help
Training and Pruning Your Home Orchard
Here is a link that might be useful: Training and Pruning Your Home Orchard
I prune all of my plums like that. It helps to keep them under control and easier to manage.
Jap plums often don't cooperate in terms of producing secondary branching where you need it off of scaffolds (main branches off the trunk). You can stub cut branches or cut back to a small shoot to encourage more of this branching, but it is best to wait until the tree is growing for the best response- that is to get best production of smaller secondary branches.
It is also very helpful to come back a few weeks later and lightly pinch back all the shoots that emerge right behind the cut besides the one that will be the new leader. You select the shoot growing closest to the angle the branch is already growing to be the leader of the branch.
Otherwise you are likely to get most of your response at the point of the cut with equal sized branches competing with each other, growing at crazy angles. Each stub cut can only get you a couple new secondary branches at the point of the cut and these secondary branches must be subdued early on to insure subdominance to the leader.