My apologies if we have veered off track. To get back on topic, I would reiterate that adding a grey tank to a 70's era GT isn't terribly difficult, and that if one chooses the VTS tanks, they fit nicely inside the frame as pictured above.
Some modifications are required, namely, one must devise a means of support for the tanks, whether it is straps or slide in panels like I am using. I also made slight mods to two cross members so that drains could pass from tank to tank as low as possible. The center belly pan is one continuous sheet from the rear bumper to just behind the fresh tank. One option may be to remove it and then put it back all in one piece, another would be to create several sections of belly skin, as I did.
When I started working under my trailer, I stacked some 2 x 12s under the wheels to raise the trailer 3 inches to give me some more working room. I also got an angle drill, which was helpful for drilling out the rivets in the belly pan. Get ready for all kinds of nasty to fall in your face as you work.