Here is the Koenig/Kamm wind tunnel development model of the FKFS.
You can see how they made the provision to truncate the boat tail at a number of locations.
Here is Heald's car of 1933.The roofline is too fast and he only scores Cd 0.20
Here is the Schl'o'rwagen,with more relaxed roofline and Cd 0.186
Here is one of Buchheim et al.'s VW aero models from 1981.
With the proper loft to the roofline,and Kamm's signature 'chop',they score Cd 0.15.
If you'll stay on the 'template' contour and then just slice off the tail where necessary,you'll have guaranteed attached flow.
