My saddle tanks are like that but the are mounted under the bed in the same spot. You can tell the tanks on my truck are purpose made for a Ford with the way they are shaped. There is even a tab on each tank in the rear to act as a fender bracket. There are two long bolts that go through each tank.
I'm redoing my interior now and I really like the factory gas tank out of the truck. I've had the saddle tanks disconnected for a while and was going to remove them since they're outside the frame rails. I might leave the factory tank out for a while and just run the driver's side saddle tank.
Falcon Ranchero
That's a pretty impressive, original truck. I would have really struggled with improving it or leaving it alone. I guess the tailgate was sort of a clincher. If it was good I would have definitely left it alone. But I can see why you did what you did now. Your work look fabulous, by the way. You didn't try to paint all the frame, did you? I presume the new black paint stops at the firewall, right? I'd like to see more exterior and under-hood "before" shots, if you have any.