I do not have the exact measurement for the placement of the end of the tips extending beyond the back of the car. I have one factory pic of a side profile where a tip shows and the tip is less extended than I mount them on the Jetfires I have restored. Many F-85’s, including Jetfires, that I have seen in salvage yards over the years have the back gravel shield rusted away in the exhaust outlet area, so I extend the tips slightly further out than the factory did.
In the attached pic of one of my cars, the end of the tip is 4 & 1/2″ out from the inside edge of the gravel shield. It seams to be a good location for function and appearance.