Well, a G body already has it's subframes connected since it is a composite body on frame construction, not a unibody like the Mustangs and Civics you have seen. Even so, the connection is done with a somewhat flimsy C channel that may flex more without the roof to strengthen the body structure. You may be able to just replace all the body bushings with polyurethane to make things more solid. However, the best thing to do would be to box in the frame's C channel like GM did on the A body convertibles and some big block cars (like the Skylark GS) in the 60's and 70's. It would take some doing to make it work well in a driver quality car. Sealing it from rain, etc would be the biggest challenge, as even GM did not seem to be able to do this right on the factory T top cars in the 70's and 80's. For a show and occasional use car, you could get by with something that looks nice but does not function perfectly since it would be garaged most of the time.Just remember that water likes to go everywhere it can, and will find even the most minor of flaws in your sealing attempts. Go to the junkyard and look at a T top car to see what GM tried, and base your attempt on their work.
To look at a factory Targa top, go through the import section and try to find an old Porsche 914 or 911 from the 70's or 80's as all 914's and some 911's came with them. They were designed by Germans, so you know they probably worked. There were also some VERY rare Toyota Celica's and AMC Concord's with what I believe was called a "Sunchaser" top. In essence, it had a targa bar for a B pillar, with a removable section over the front seat, and a folding convertible top over the back. Then there is also the Fiat X/19 from the same period that also had a Targa roof. That's all of the factory targa roofs I can think of off hand that did not come on a Ferrari or a Corvette.
