The Template is what you call an 'attractive nuisance'. Look at the front-rear view and you see it is a half-circle. It has almost no application this side of extreme solar/human powered race cars. The closest you can come is like the MG 181.
http://ecomodder.com/forum/member-sven7-albums-aero+vehicles-picture3242-mg-ex181-streamliner.html
What you can take away is the relaxed, curved taper to stall separation as long as possible. Your truncation is more of a Kamm-back than a boattail, and would have minimal effect. You could take the existing cross-section at the rear and reduce it stepwise along the Template curve.
You might see more return from a rear partial bellypan, it depends. If you went to the next dotted white line, with side tapers balancing (or counter-balancing) the top taper and dealt with the taillights and hatch access, you should start to see some measurable results. Here are some box cavity studies as an alternative:
