they are the same in length and over all diameter but the wheel position is differant so you can not just put it on the truck. Sorry bro they are not interchangable with out some fab work.
When you say double cab do you mean a 4 door. or just a plan run of the mill ex cab (just 2 doors). Cause if it is a true Double cab then the bed is way shorter
I just went out and measured my 2 trucks because I was curious. My '87 extended cab bed is 74.5 inches and my '87 standard cab bed is 76 inches. You learn something new every day.