Identifying the curve is the easy part. Fit a parabola to the streamlining template above to create an equation for its curvature. Place the origin at the point of max. roof camber where x=0, y=0, y'=0, and knowing the point (1.78d,-.5d), the curve becomes y(x)=-.157808*x/d.
Since you're in Arkansas, I'll assume you have been unable to find a metric tape measure, so here's the equation for your sides (d=74.8in) in inches: y=.00211x²
and for the top (d=140.2in): y=.001126x²
Plug in x, the number of inches past the start of the curvature, and it spits out y, the number of inches inward your curvature has moved. I chose to plot points every few inches and cut out the curve in cardboard. Both the negative and the positive cardboard templates are useful for checking your curvature while building the boat-tail.
Now, how are you going to manufacture this beast? People have created body bucks in wire, which is easy enough to bend into a parabola.
I've been working in styrofoam, which has its merits and drawbacks. You could also do a quick approximation of the ideal curvature using flat sheets of coroplast or sheet metal, screwed or riveted to a metal frame. This method here is pretty attractive:
