You didn't like the advice you got on WSO?
Responding to a few other comments:
I honestly don't think ORFE (undergrad) is that narrow. You can do anything from an Econ to a CS PhD with it; given a Princeton ORFE degree and a decent GPA, you can find jobs anywhere from Google to WalMart to Goldman in that field.
Undergrad econ, even from Harvard, cuts off a lot of opportunities. You will *probably* never be an engineer. You will *probably* never work as a developer at a tech firm. You will not be a Jeff Bezos or an Elon Musk, which is a tail case but not completely ridiculous going to Harvard or Princeton. If that's not what you want, it's not a big deal, but you may decide that you truly enjoy programming or engineering. With the undergrad econ degree, it will be too late.
Careers and educations tend to move from more quantitative to less quantitative. You see math majors sometimes do CS or engineering for grad school, but you don't see a lot of Finance majors jump (gracefully) to CS. You don't see as many Econ majors switch to Engineering. You can always go to a softer discipline for grad school- or do an MBA- but I would lean more towards an engineering undergrad assuming you can handle the math.
I would lean away from taking MIT math courses if you're going to Harvard. There are the Harvard and Princeton math people... and then there are the MIT and Caltech math people who scare the bejeezus out of the H/P folks. Regardless of your standardized test scores, there are pretty darned smart people who get perfect SAT scores, and then there are downright geniuses who get perfect SAT scores. You don't want to be competing against those people for an A; heck, you don't want to be competing against those people for a B-.