Here is a picture of the masonite board from a 67 Dearborn built car. One side is smooth, and one was rough. I assume that the rough side was on the tank, based on rust stains. The thickness measures 3/16" but was probably 1/8" originally and is separating due to age and/or water exposure. I have no knowledge or reference data for MY 68. This is simply a single reference point. And we know how dangerous that can be. But for those that need it, the dimensions are 6"x12". I put a square against one of the short sides that looked off, and confirmed it was not square. I could not make out any saw cuts.