Torchie has a great idea, but you also need to pay attention close attention to the 3D curves and if the sides curve inward at all.
Basically when you photochop a car, you're doing the same thing, just with a PC instead of CB. (carboard)
Anyhow, this is how I might go about that wedge chop:
The A posts, just cut the top off.
The doors, cut somewhere in the middle, and the angle relief cuts in the corners. The top may need to be cut and lengthened slightly possibly using some of the metal from the front or back that was removed.
The back, cut right below the curved corner and then up to the top corner of the door opening. Weld the flat back section in and fold the roof forward. The vertical relief cut would follow the curve of the corner and curve forward at the same time to avoid a sharp point when folding it.
There might me a little work to do up front on the roof section since it will no longer quite reach the A-pillar.