Yep, it's right there in the tail portion of the frame. The tank will fit in there, but the cross members on a truck are a bit different than the RC. There is a crossmember on the trucks that you might need to modify or remove. Hard to explain, but I will try to get a pic for you. I ran into this on my RC conversion. I WAS using a '91 truck frame that i had shortend to fit a '78 RC body. The cross members for the fuel tank were being a pain, but I'm sure it can be done. Maybe take a look for an older('78 ish) 3/4 ton that had dual tanks, from the factory. I have one of the rear tanks from one of them and it would have worked great but my tank had a big cut in the top of it. They exist though. Just a thought.
Andrew