I've done it a few times, some pics in my gallery of a chebbie pickup we stretched 7". Easiest way is to find a spare set of doors. Use the front of the doors you have, cut them in a place where all the lines are straight, then cut the back portion of the donor doors at the point where the length matches the extension you want. Butt the two doors together. A flanged joint on the door will strengthen the skin considerably and making the welding a lot easier. Once you get the doors back on the hinges, cut the cab and fit the door jamb to the door. Before you cut the cab, make sure it is very well braced on the inside. Things move around a lot when you start cutting.....