The commerce, the population, the money - most of it is in the east. Boston is by far the most probable location. If it's there, it's probably the capital.
On the other hand, what if it isn't there? Suppose that when Howe withdrew from Boston in 1776, he just flattens the city - completely demolishes it in an act of scorched earth. He wasn't that sort of man, as far as I know, nor would it have been good policy, but suppose. The port's ruined, burned, blockaded - valueless. The capital moves west (pick a spot). Maybe it stays there, or maybe the British come back circa 1812 and do it again, after which the shift is made permanent.