Well, I took some measurements on the frame today and I can say beyond the shadow of a doubt that the rear frame section of a 4 door where the gas tank sits is pretty much exactly the same as a 2 door, just slightly longer(just over 5" front to back). Outside left to right dimension is 34", inside is 29-3/4". Mounting tabs for the tank are |removethispart|@19" center to center from left to right both in front of the tank and behind it. The inside measure between the rear cross member and the one just in front of the tank is 20" on the 4 door, 14-3/4" on the 2 door.
The 4 door tank is longer front to back than the 2 door tank and also slightly wider left to right. However the pinch welds on the 4 door tank are wider than the frame opening which is why the 4 door tank can't be lifted without some work to bend the pinch weld over or cut it off. Or if your body mount is high enough, you might be able to set the pinch welds on top of the frame rails, but you'd need a 4 or 5" body lift at least...
I'm not sure which tank I'm going to use in my project yet, both are in servicable condition. But I'll figure that out later. Next up is removing everything else from the frame, Basically everything that will unbolt, then I need to scrape off all the old undercoating, strip the paint and rust, weld on the reinforcements and repaint it. Then start re-finishing the suspension components and put them back on.