Your GMAT score is a bit low, yes, but I worked with a client last year who was admitted to the 2+2 program with the same score (maybe even 10 points lower). He also had similar grades, major, etc. You know what my biggest concern was when we started? Not GMAT score - it was the fact that he was such a "business major." This is not because I was a PR major (aka Common Sense major) in college that I put it in quotes, as if to suggest disdain, but rather to call out the fact that HBS built this program to attract everything but business majors. In fact, they used to restrict the program from anyone with a business background. You have to understand that the 2+2 program was designed to attract young talent that was headed elsewhere (Kennedy School, Harvard Law, Capital Hill, Julliard, you name it) and route them to HBS. If you look like someone who will just apply to HBS in three years anyway, why should they bother, right?
So they key with my former client - and with you - will be to focus on the following:
1. Diversity of perspective. Show what a 360 degree thinker you are. Take advantage of the fact you are still in college and no doubt just finished a general education program as well as a major, and dig into language, philosophy, the arts, and so on as a motivator for how you think and what your pursuits are. Cherry pick accomplishments, setbacks, and experiences that aren't what you think HBS wants to hear - pretend you are applying to Harvard Medical School to be a doctor in a rural community or Harvard Law School to be a public interest lawyer. It will free your mind to get out of "think like an MBA" and instead "think like a young, bright mind." They want the latter.
2. Readiness. EQ, maturity, etc. Critical. The biggest way to do this is to be wide open, transparent, and introspective on your setbacks.
3. WHY NOW. This is the big one. You simply HAVE to explain to them why 2+2 is something uniquely good for both parties. A) why shouldn't you just wait a few years and apply then like everyone else? B) why shouldn't HBS just wait and see how you pan out? It really makes no sense for them to hand you a spot and assume/hope that you will do well in the real world. So you have to make a strong case for why this is a good decision. What does HBS gain by including you in this small group, locked in for the near future? What do YOU gain (other than "I got into Harvard!" by securing this now? This is the magic ingredient in the recipe.
Anyway, I hope this liberates your fears about the GMAT and sharpens your focus on what you have to do.