I am not sure (again, layman speaking!) that it is possible at all to "take the text of 18th century and the philosophy of the time of the text" without being arbitrary. "Philosophy" - being what? If the protections afforded by the Bill of Rights at the time of its inception constituted the most revolutionary philosophy of the day, should the courts of 1920s have been guided by the most revolutionary theory if their day, i.e. Marxism-Leninism?

I mean, mixing "philosophy" in is, essentially, evaluating a degree to which this philosophy was "good" from YOUR point of view. You would say that the philosophy of the Founders was to create the most just country in the world, given the limitations of the time; hence it implies equality and other nice things. However, enter a neo-Nazi, and he would argue that the philosophy of the Founders was to afford protection to white male property owners, so the coloreds are screwed, etc. - and from the formal point of view his theory is as valid as yours. Philosophy, to me, seems a shaky basis to build on.