[aerohead]

The Caprice,as a body-on-frame structure is enticing unless you wanted the extra height( forward visibility) of the pickup cab.
Without any tail,but short nose and comprehensive streamlining you'd be looking at Cd 0.19 territory,no matter who's truck you start with.
With boat-tailing,the Cd will begin to fall off as you add length.
My T-100 started at Cd 0.44 and with the 'kit',and 30" of tail on it,the mpg/Cd relationship published by GM suggests Cd 0.168.
Finishing out the tail might get her to 0.12-13.I hope to have those numbers by next September.
When Hucho was at Volkswagen,they tried really hard to get the Golf/Rabbit drag down by smoothing the front and after just a tiny bit of rounding saw no real benefit with extreme radii.
They did see real drag reduction when they 'Templated' the rear end.
My opinion would be to forget about the Caprice and go directly to the pickup.
Hucho's own pumpkin seed 'car' of 1978 achieved Cd 0.15,and that was with an 'illegal' rear contour,and nothing done to fair in the wheels.