31 inches in the back and 27 inches in the front will be the best you can do. I speak from experience from the 2 player games I own (have owned) from that era with the funky "lip" in the front. 1964 Gottlieb Big Top and 1964 Gottlieb Ship Mates is my other example.
EDIT: I just measured out the front legs on Ship Mates. They are 28 1/2 inches long. I would just order the 1960's Bally 28 1/2 inch legs if you can't find original Gottlieb ones (good luck on that)
I am starting to think my Ship Mates legs are not original. I do remember measuring the Big Top legs as 27" (again I don't think they were original though)