Just based on what I’ve learned from one child who went through the engineering admissions process, and another who just took the Math 2 subject test, plus looking at the CB statistics, I’d say at least 700, and preferably 730 or more.
My take on top schools saying they’ll take either is like them saying Subject Tests are optional. I think submitting anything under an 800 Math 1 to MIT, and even an 800, would put you at a disadvantage.