A DS boom should be 10' long (more or less). Discovering yours is 2ft short must have been a bit of unwelcome surprise. Makes you wonder what happened to the original boom?
I would go for replacing the boom. Fairly soon, you won't be a beginner any more, and unless you sail very infrequently, there will come the time that you need (or want) to replace your sails. With the wrong sized boom you are sailing with a permanently "reefed" main and you will need to repeat whatever custom sail solution you are thinking of doing now.
DR Marine sells new booms, but you may be able to pick up a used one.
A smaller mainsail also affects the balance of your sail plan, unless you were to get an undersized jib to go along with it. The mainsail sits behind the "pivot" location of the boat which is defined by where the centerboard sits. So, wind in the main will drive the boat to turn upwind. Conversely, wind in the jib (forward of the CB) will drive the boat down.
An ideal balance for a boat is to have just a bit of "weather helm" that is, fairly balanced but with a slight tendency to round up when you let go of the tiller. The opposite, "lee helm" is generally considered bad. That's what you get when the jib's contribution is the dominant one and drives the bow down. It will make tacking more difficult for example.
So, even if you are a beginner, sailing with an unbalanced rig will do you no favors.
PS: have you checked out the "
Basic Concepts and Techniques" thread yet?