This car was up about 6 months ago.

If I was going to build a pickup from a car, I'd cut just behind the front doors, use a cab back from a mini truck, fab a frame out the back for the rear suspension and make a "looks like a flat bed but has minimal wheel wells in the bed and fold down sides. "

Welcome to*Ute Ltd - Manufacturers of Tough Aluminum Pick-up Truck Beds

It would look like this but I'd make taller sides and push the wheels up into the deck a bit though.

http://www.uteltd.com/PDF/gm/2016GM-Dealer-blank.pdf