I think there is someone making AD under bed tanks now but they are so simple we make our own. Right off the top of my head I think they are 13x13x21 inches. The pickup tube needs to be built in plus the filler neck and sending unit plates for additional backing strength. You also need channel or angle to make the tank to frame brackets but they are a simple "L" shape. We had a run of 48 pickups and several Suburbans so tank making became a necessity. I am VERY poor at posting pictures with text but will give it a try. If all fails maybe I can email the pics to someone skilled (Like Tim) who can then post them. PICTURE # 1 is the very basic tools needed (plus a welder). The pipe on the edge of the welding table is spaced up the thickness of a washer and then tacked to the table. This gives a slot to slide the 18g cold rolled metal through and make nice looking rounded corners but square corners would be fine. PICTURE #2 is the pickup tube tacked to bottom for rigidity. PICTURE #3 is the pickup fitting at same location as a factory tank. PICTURE #4 is a completed tank ready for filler and sending unit. Before welding final end in place you can use a baffle but the factory didn't.

http://s32.photobucket.com/user/coilover/media/underbed%20gas%20tank%20004.jpg.html
http://s32.photobucket.com/user/coilover/media/underbed%20gas%20tank%20006_1.jpg.html
http://s32.photobucket.com/user/coilover/media/underbed%20gas%20tank%20005.jpg.html
http://s32.photobucket.com/user/coilover/media/underbed%20gas%20tank%20003.jpg.html

Evan