I typically add 55-60 oz of canned pumpkin puree to my pumpkin ales. I don't adjust the grain bill at all, as the pumpkin does not add any appreciable amount of fermentables. I have also never worried about mash pH. According to a Google search, cooked pumpkin meat (canned pumpkin is already cooked) typically has a pH of 4.8-5.2, so it would only help with proper mash pH.
Good luck!